Preface

Working with object-oriented software and a relational database can be cumbersome
and time consuming in today's enterprise environments. Hibernate is an Object/Relational
Mapping tool for Java environments. The term Object/Relational Mapping (ORM) refers to
the technique of mapping a data representation from an object model to a relational
data model with a SQL-based schema.

Hibernate not only takes care of the mapping from Java classes to
database tables (and from Java data types to SQL data types), but also provides data
query and retrieval facilities. It can also significantly reduce development time otherwise
spent with manual data handling in SQL and JDBC.

Hibernate's goal is to relieve the developer from 95 percent of common data persistence
related programming tasks. Hibernate may not be the best solution for data-centric
applications that only use stored-procedures to implement the business logic in the
database, it is most useful with object-oriented domain models and business logic in
the Java-based middle-tier. However, Hibernate can certainly help you to remove or
encapsulate vendor-specific SQL code and will help with the common task of result set
translation from a tabular representation to a graph of objects.

If you are new to Hibernate and Object/Relational Mapping or even Java,
please follow these steps:

Read Chapter 1, Tutorial for a tutorial with step-by-step
instructions. The source code for the tutorial is included in the
distribution in the doc/reference/tutorial/
directory.

View the eg/ directory in the Hibernate
distribution. It contains a simple standalone application. Copy your
JDBC driver to the lib/ directory and edit
etc/hibernate.properties, specifying correct values for
your database. From a command prompt in the distribution directory,
type ant eg (using Ant), or under Windows, type
build eg.

Use this reference documentation as your primary source of
information. Consider reading [JPwH]
if you need more help with application design, or if you prefer
a step-by-step tutorial. Also visit
http://caveatemptor.hibernate.org and download
the example application from [JPwH].

FAQs are answered on the Hibernate website.

Links to third party demos, examples, and tutorials are maintained
on the Hibernate website.

The Community Area on the Hibernate website is a good resource for
design patterns and various integration solutions (Tomcat, JBoss AS,
Struts, EJB, etc.).

If you have questions, use the user forum linked on the Hibernate website. We also
provide a JIRA issue tracking system for bug reports and feature requests. If you
are interested in the development of Hibernate, join the developer mailing list. If
you are interested in translating this documentation into your language, contact us
on the developer mailing list.

Commercial development support, production support, and training for Hibernate is
available through JBoss Inc. (see http://www.hibernate.org/SupportTraining/).
Hibernate is a Professional Open Source project and a critical component of the
JBoss Enterprise Middleware System (JEMS) suite of products.

Intended for new users, this chapter provides an step-by-step introduction
to Hibernate, starting with a simple application using an in-memory database. The
tutorial is based on an earlier tutorial developed by Michael Gloegl. All
code is contained in the tutorials/web directory of the project
source.

Important

This tutorial expects the user have knowledge of both Java and
SQL. If you have a limited knowledge of JAVA or SQL, it is advised
that you start with a good introduction to that technology prior
to attempting to learn Hibernate.

Note

The distribution contains another example application under
the tutorial/eg project source
directory.

1.1. Part 1 - The first Hibernate Application

For this example, we will set up a small database application that can store
events we want to attend and information about the host(s) of these events.

Note

Although you can use whatever database you feel comfortable using, we
will use HSQLDB (an in-memory,
Java database) to avoid describing installation/setup of any particular
database servers.

1.1.1. Setup

The first thing we need to do is to set up the development environment. We
will be using the "standard layout" advocated by alot of build tools such
as Maven. Maven, in particular, has a
good resource describing this layout.
As this tutorial is to be a web application, we will be creating and making
use of src/main/java, src/main/resources
and src/main/webapp directories.

We will be using Maven in this tutorial, taking advantage of its
transitive dependency management capabilities as well as the ability of
many IDEs to automatically set up a project for us based on the maven descriptor.

<projectxmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>org.hibernate.tutorials</groupId><artifactId>hibernate-tutorial</artifactId><version>1.0.0-SNAPSHOT</version><name>First Hibernate Tutorial</name><build><!-- we dont want the version to be part of the generated war file name --><finalName>${artifactId}</finalName></build><dependencies><dependency><groupId>org.hibernate</groupId><artifactId>hibernate-core</artifactId></dependency><!-- Because this is a web app, we also have a dependency on the servlet api. --><dependency><groupId>javax.servlet</groupId><artifactId>servlet-api</artifactId></dependency><!-- Hibernate uses slf4j for logging, for our purposes here use the simple backend --><dependency><groupId>org.slf4j</groupId><artifactId>slf4j-simple</artifactId></dependency><!-- Hibernate gives you a choice of bytecode providers between cglib and javassist --><dependency><groupId>javassist</groupId><artifactId>javassist</artifactId></dependency></dependencies></project>

Tip

It is not a requirement to use Maven. If you wish to use something else to
build this tutorial (such as Ant), the layout will remain the same. The only
change is that you will need to manually account for all the needed
dependencies. If you use something like Ivy
providing transitive dependency management you would still use the dependencies
mentioned below. Otherwise, you'd need to grab all
dependencies, both explicit and transitive, and add them to the project's
classpath. If working from the Hibernate distribution bundle, this would mean
hibernate3.jar, all artifacts in the
lib/required directory and all files from either the
lib/bytecode/cglib or lib/bytecode/javassist
directory; additionally you will need both the servlet-api jar and one of the slf4j
logging backends.

Save this file as pom.xml in the project root directory.

1.1.2. The first class

Next, we create a class that represents the event we want to store in the
database; it is a simple JavaBean class with some properties:

This class uses standard JavaBean naming conventions for property
getter and setter methods, as well as private visibility for the
fields. Although this is the recommended design, it is not required.
Hibernate can also access fields directly, the benefit of accessor
methods is robustness for refactoring.

The id property holds a unique identifier value
for a particular event. All persistent entity classes (there are
less important dependent classes as well) will need such an identifier
property if we want to use the full feature set of Hibernate. In fact,
most applications, especially web applications, need to distinguish
objects by identifier, so you should consider this a feature rather
than a limitation. However, we usually do not manipulate the identity
of an object, hence the setter method should be private. Only Hibernate
will assign identifiers when an object is saved. Hibernate can access
public, private, and protected accessor methods, as well as public,
private and protected fields directly. The choice is up to you and
you can match it to fit your application design.

The no-argument constructor is a requirement for all persistent
classes; Hibernate has to create objects for you, using Java
Reflection. The constructor can be private, however package or public
visibility is required for runtime proxy generation and efficient data
retrieval without bytecode instrumentation.

Save this file to the src/main/java/org/hibernate/tutorial/domain
directory.

1.1.3. The mapping file

Hibernate needs to know how to load and store objects of the
persistent class. This is where the Hibernate mapping file
comes into play. The mapping file tells Hibernate what table in
the database it has to access, and what columns in that table
it should use.

Hibernate DTD is sophisticated. You can use it for auto-completion
of XML mapping elements and attributes in your editor or IDE.
Opening up the DTD file in your text editor is the easiest way to
get an overview of all elements and attributes, and to view the
defaults, as well as some comments. Hibernate will not load the
DTD file from the web, but first look it up from the classpath of
the application. The DTD file is included in
hibernate-core.jar (it is also included in the
hibernate3.jar, if using the distribution bundle).

Important

We will omit the DTD declaration in future examples to shorten the code. It is,
of course, not optional.

Between the two hibernate-mapping tags, include a
class element. All persistent entity classes (again, there
might be dependent classes later on, which are not first-class entities) need
a mapping to a table in the SQL database:

So far we have told Hibernate how to persist and load object of
class Event to the table
EVENTS. Each instance is now represented by a
row in that table. Now we can continue by mapping the unique
identifier property to the tables primary key. As we do not want
to care about handling this identifier, we configure Hibernate's
identifier generation strategy for a surrogate primary key column:

The id element is the declaration of the
identifier property. The name="id" mapping
attribute declares the name of the JavaBean property and tells
Hibernate to use the getId() and
setId() methods to access the property. The
column attribute tells Hibernate which column of the
EVENTS table holds the primary key value.

The nested generator element specifies the
identifier generation strategy (aka how are identifier values
generated?). In this case we choose native,
which offers a level of portability depending on the configured
database dialect. Hibernate supports database generated, globally
unique, as well as application assigned, identifiers. Identifier
value generation is also one of Hibernate's many extension points
and you can plugin in your own strategy.

Similar to the id element, the
name attribute of the
property element tells Hibernate which getter
and setter methods to use. In this case, Hibernate will search
for getDate(), setDate(),
getTitle() and setTitle()
methods.

Note

Why does the date property mapping include the
column attribute, but the title
does not? Without the column attribute, Hibernate
by default uses the property name as the column name. This works for
title, however, date is a reserved
keyword in most databases so you will need to map it to a different name.

The title mapping also lacks a type attribute. The
types declared and used in the mapping files are not Java data types; they are not SQL
database types either. These types are called Hibernate mapping types,
converters which can translate from Java to SQL data types and vice versa. Again,
Hibernate will try to determine the correct conversion and mapping type itself if
the type attribute is not present in the mapping. In some cases this
automatic detection using Reflection on the Java class might not have the default you
expect or need. This is the case with the date property. Hibernate cannot
know if the property, which is of java.util.Date, should map to a
SQL date, timestamp, or time column.
Full date and time information is preserved by mapping the property with a
timestamp converter.

Tip

Hibernate makes this mapping type determination using reflection when the mapping files
are processed. This can take time and resources, so if startup performance is important
you should consider explicitly defining the type to use.

Save this mapping file as
src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml.

1.1.4. Hibernate configuration

At this point, you should have the persistent class and its mapping
file in place. It is now time to configure Hibernate. First let's set up
HSQLDB to run in "server mode"

Note

We do this do that the data remains between runs.

We will utilize the Maven exec plugin to launch the HSQLDB server
by running:
mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/tutorial"
You will see it start up and bind to a TCP/IP socket; this is where
our application will connect later. If you want to start
with a fresh database during this tutorial, shutdown HSQLDB, delete
all files in the target/data directory,
and start HSQLDB again.

Hibernate will be connecting to the database on behalf of your application, so it needs to know
how to obtain connections. For this tutorial we will be using a standalone connection
pool (as opposed to a javax.sql.DataSource). Hibernate comes with
support for two third-party open source JDBC connection pools:
c3p0 and
proxool. However, we will be using the
Hibernate built-in connection pool for this tutorial.

Caution

The built-in Hibernate connection pool is in no way intended for production use. It
lacks several features found on any decent connection pool.

For Hibernate's configuration, we can use a simple hibernate.properties file, a
more sophisticated hibernate.cfg.xml file, or even complete
programmatic setup. Most users prefer the XML configuration file:

Note

Notice that this configuration file specifies a different DTD

You configure Hibernate's SessionFactory. SessionFactory is a global
factory responsible for a particular database. If you have several databases, for easier
startup you should use several <session-factory> configurations in
several configuration files.

The first four property elements contain the necessary
configuration for the JDBC connection. The dialect property
element specifies the particular SQL variant Hibernate generates.

Tip

Hibernate's automatic session management for persistence contexts is particularly useful
in this context. The hbm2ddl.auto option turns on automatic generation of
database schemas directly into the database. This can also be turned
off by removing the configuration option, or redirected to a file with the help of
the SchemaExport Ant task. Finally, add the mapping file(s)
for persistent classes to the configuration.

Save this file as hibernate.cfg.xml into the
src/main/resources directory.

1.1.5. Building with Maven

We will now build the tutorial with Maven. You will need to
have Maven installed; it is available from the
Maven download page.
Maven will read the /pom.xml file we created
earlier and know how to perform some basic project tasks. First,
lets run the compile goal to make sure we can compile
everything so far:

1.1.6. Startup and helpers

It is time to load and store some Event
objects, but first you have to complete the setup with some
infrastructure code. You have to startup Hibernate by building
a global org.hibernate.SessionFactory
object and storing it somewhere for easy access in application code. A
org.hibernate.SessionFactory is used to
obtain org.hibernate.Session instances.
A org.hibernate.Session represents a
single-threaded unit of work. The
org.hibernate.SessionFactory is a
thread-safe global object that is instantiated once.

We will create a HibernateUtil helper class that
takes care of startup and makes accessing the
org.hibernate.SessionFactory more convenient.

Save this code as
src/main/java/org/hibernate/tutorial/util/HibernateUtil.java

This class not only produces the global
org.hibernate.SessionFactory reference in
its static initializer; it also hides the fact that it uses a
static singleton. We might just as well have looked up the
org.hibernate.SessionFactory reference from
JNDI in an application server or any other location for that matter.

If you give the org.hibernate.SessionFactory
a name in your configuration, Hibernate will try to bind it to
JNDI under that name after it has been built. Another, better option is to
use a JMX deployment and let the JMX-capable container instantiate and bind
a HibernateService to JNDI. Such advanced options are
discussed later.

You now need to configure a logging
system. Hibernate uses commons logging and provides two choices: Log4j and
JDK 1.4 logging. Most developers prefer Log4j: copy log4j.properties
from the Hibernate distribution in the etc/ directory to
your src directory, next to hibernate.cfg.xml.
If you prefer to have
more verbose output than that provided in the example configuration, you can change the settings. By default, only the Hibernate startup message is shown on stdout.

The tutorial infrastructure is complete and you are now ready to do some real work with
Hibernate.

1.1.7. Loading and storing objects

We are now ready to start doing some real work with Hibernate.
Let's start by writing an EventManager class
with a main() method:

In createAndStoreEvent() we created a new
Event object and handed it over to Hibernate.
At that point, Hibernate takes care of the SQL and executes an
INSERT on the database.

A org.hibernate.Session is designed to
represent a single unit of work (a single atomic piece of work
to be performed). For now we will keep things simple and assume
a one-to-one granularity between a Hibernate
org.hibernate.Session and a database
transaction. To shield our code from the actual underlying
transaction system we use the Hibernate
org.hibernate.Transaction API.
In this particular case we are using JDBC-based transactional
semantics, but it could also run with JTA.

What does sessionFactory.getCurrentSession() do?
First, you can call it as many times and anywhere you like
once you get hold of your
org.hibernate.SessionFactory.
The getCurrentSession() method always returns
the "current" unit of work. Remember that we switched
the configuration option for this mechanism to "thread" in our
src/main/resources/hibernate.cfg.xml?
Due to that setting, the context of a current unit of work is bound
to the current Java thread that executes the application.

Important

Hibernate offers three methods of current session tracking.
The "thread" based method is not intended for production use;
it is merely useful for prototyping and tutorials such as this
one. Current session tracking is discussed in more detail
later on.

A org.hibernate.Session begins when the
first call to getCurrentSession() is made for
the current thread. It is then bound by Hibernate to the current
thread. When the transaction ends, either through commit or
rollback, Hibernate automatically unbinds the
org.hibernate.Session from the thread
and closes it for you. If you call
getCurrentSession() again, you get a new
org.hibernate.Session and can start a
new unit of work.

Related to the unit of work scope, should the Hibernate
org.hibernate.Session be used to execute
one or several database operations? The above example uses one
org.hibernate.Session for one operation.
However this is pure coincidence; the example is just not complex
enough to show any other approach. The scope of a Hibernate
org.hibernate.Session is flexible but you
should never design your application to use a new Hibernate
org.hibernate.Session for
every database operation. Even though it is
used in the following examples, consider
session-per-operation an anti-pattern.
A real web application is shown later in the tutorial which will
help illustrate this.

Here, we are using a Hibernate Query Language (HQL) query to load all existing
Event objects from the database. Hibernate will generate the
appropriate SQL, send it to the database and populate Event objects
with the data. You can create more complex queries with HQL. See Chapter 15, HQL: The Hibernate Query Language
for more information.

Now we can call our new functionality, again using the Maven exec plugin:
mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"

1.2. Part 2 - Mapping associations

So far we have mapped a single persistent entity class to a table in
isolation. Let's expand on that a bit and add some class associations.
We will add people to the application and store a list of events in
which they participate.

Create an association between these two entities. Persons
can participate in events, and events have participants. The design questions
you have to deal with are: directionality, multiplicity, and collection
behavior.

1.2.2. A unidirectional Set-based association

By adding a collection of events to the Person
class, you can easily navigate to the events for a particular person,
without executing an explicit query - by calling
Person#getEvents. Multi-valued associations
are represented in Hibernate by one of the Java Collection Framework
contracts; here we choose a java.util.Set
because the collection will not contain duplicate elements and the ordering
is not relevant to our examples:

Before mapping this association, let's consider the other side.
We could just keep this unidirectional or create another
collection on the Event, if we wanted to be
able to navigate it from both directions. This is not necessary,
from a functional perspective. You can always execute an explicit
query to retrieve the participants for a particular event. This
is a design choice left to you, but what is clear from this
discussion is the multiplicity of the association: "many" valued
on both sides is called a many-to-many
association. Hence, we use Hibernate's many-to-many mapping:

Hibernate supports a broad range of collection mappings, a
set being most common. For a many-to-many
association, or n:m entity relationship, an
association table is required. Each row in this table represents
a link between a person and an event. The table name is
decalred using the table attribute of the
set element. The identifier column name in
the association, for the person side, is defined with the
key element, the column name for the event's
side with the column attribute of the
many-to-many. You also have to tell Hibernate
the class of the objects in your collection (the class on the
other side of the collection of references).

After loading a Person and an
Event, simply modify the collection using the
normal collection methods. There is no explicit call to
update() or save();
Hibernate automatically detects that the collection has been modified
and needs to be updated. This is called
automatic dirty checking. You can also try
it by modifying the name or the date property of any of your
objects. As long as they are in persistent
state, that is, bound to a particular Hibernate
org.hibernate.Session, Hibernate
monitors any changes and executes SQL in a write-behind fashion.
The process of synchronizing the memory state with the database,
usually only at the end of a unit of work, is called
flushing. In our code, the unit of work
ends with a commit, or rollback, of the database transaction.

You can load person and event in different units of work. Or
you can modify an object outside of a
org.hibernate.Session, when it
is not in persistent state (if it was persistent before, this
state is called detached). You can even
modify a collection when it is detached:

privatevoid addPersonToEvent(Long personId,Long eventId){Session session =HibernateUtil.getSessionFactory().getCurrentSession(); session.beginTransaction();Person aPerson =(Person) session.createQuery("select p from Person p left join fetch p.events where p.id = :pid").setParameter("pid", personId).uniqueResult();//Eager fetch the collection so we can use it detachedEvent anEvent =(Event) session.load(Event.class, eventId); session.getTransaction().commit();//End of first unit of work aPerson.getEvents().add(anEvent);// aPerson (and its collection) is detached//Begin second unit of workSession session2 =HibernateUtil.getSessionFactory().getCurrentSession(); session2.beginTransaction(); session2.update(aPerson);//Reattachment of aPerson session2.getTransaction().commit();}

The call to update makes a detached object
persistent again by binding it to a new unit of work, so any
modifications you made to it while detached can be saved to
the database. This includes any modifications
(additions/deletions) you made to a collection of that entity
object.

This is not much use in our example, but it is an important concept you can
incorporate into your own application. Complete this exercise by adding a new action
to the main method of the EventManager and call it from the command line. If
you need the identifiers of a person and an event - the save() method
returns it (you might have to modify some of the previous methods to return that identifier):

This is an example of an association between two equally important
classes : two entities. As mentioned earlier, there are other
classes and types in a typical model, usually "less important".
Some you have already seen, like an int or a
java.lang.String. We call these classes
value types, and their instances
depend on a particular entity. Instances of
these types do not have their own identity, nor are they shared
between entities. Two persons do not reference the same
firstname object, even if they have the same
first name. Value types cannot only be found in the JDK , but
you can also write dependent classes yourself
such as an Address or
MonetaryAmount class. In fact, in a Hibernate
application all JDK classes are considered value types.

You can also design a collection of value types. This is
conceptually different from a collection of references to other
entities, but looks almost the same in Java.

1.2.4. Collection of values

Let's add a collection of email addresses to the
Person entity. This will be represented as a
java.util.Set of
java.lang.String instances:

The difference compared with the earlier mapping is the use of
the element part which tells Hibernate that the
collection does not contain references to another entity, but is
rather a collection whose elements are values types, here specifically
of type string. The lowercase name tells you
it is a Hibernate mapping type/converter. Again the
table attribute of the set
element determines the table name for the collection. The
key element defines the foreign-key column
name in the collection table. The column
attribute in the element element defines the
column name where the email address values will actually
be stored.

You can see that the primary key of the collection table is in fact a composite key that
uses both columns. This also implies that there cannot be duplicate email addresses
per person, which is exactly the semantics we need for a set in Java.

You can now try to add elements to this collection, just like we did before by
linking persons and events. It is the same code in Java:

This time we did not use a fetch query to
initialize the collection. Monitor the SQL log and try to
optimize this with an eager fetch.

1.2.5. Bi-directional associations

Next you will map a bi-directional association. You will make
the association between person and event work from both sides
in Java. The database schema does not change, so you will still
have many-to-many multiplicity.

Note

A relational database is more flexible than a network
programming language, in that it does not need a navigation
direction; data can be viewed and retrieved in any possible
way.

These are normal set mappings in both mapping documents.
Notice that the column names in key and many-to-many
swap in both mapping documents. The most important addition here is the
inverse="true" attribute in the set element of the
Event's collection mapping.

What this means is that Hibernate should take the other side, the Person class,
when it needs to find out information about the link between the two. This will be a lot easier to
understand once you see how the bi-directional link between our two entities is created.

1.2.6. Working bi-directional links

First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a
link between a Person and an Event in the unidirectional
example? You add an instance of Event to the collection of event references,
of an instance of Person. If you want to make this link
bi-directional, you have to do the same on the other side by adding a Person
reference to the collection in an Event. This process of "setting the link on both sides"
is absolutely necessary with bi-directional links.

Many developers program defensively and create link management methods to
correctly set both sides (for example, in Person):

The get and set methods for the collection are now protected. This allows classes in the
same package and subclasses to still access the methods, but prevents everybody else from altering
the collections directly. Repeat the steps for the collection
on the other side.

What about the inverse mapping attribute? For you, and for Java, a bi-directional
link is simply a matter of setting the references on both sides correctly. Hibernate, however, does not
have enough information to correctly arrange SQL INSERT and UPDATE
statements (to avoid constraint violations). Making one side of the association inverse tells Hibernate to consider it a mirror of the other side. That is all that is necessary
for Hibernate to resolve any issues that arise when transforming a directional navigation model to
a SQL database schema. The rules are straightforward: all bi-directional associations
need one side as inverse. In a one-to-many association it has to be the many-side,
and in many-to-many association you can select either side.

1.3. Part 3 - The EventManager web application

A Hibernate web application uses Session and Transaction
almost like a standalone application. However, some common patterns are useful. You can now write
an EventManagerServlet. This servlet can list all events stored in the
database, and it provides an HTML form to enter new events.

1.3.1. Writing the basic servlet

First we need create our basic processing servlet. Since our
servlet only handles HTTP GET requests, we
will only implement the doGet() method:

Save this servlet as
src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java

The pattern applied here is called session-per-request.
When a request hits the servlet, a new Hibernate Session is
opened through the first call to getCurrentSession() on the
SessionFactory. A database transaction is then started. All
data access occurs inside a transaction irrespective of whether the data is read or written.
Do not use the auto-commit mode in applications.

Do not use a new Hibernate Session for
every database operation. Use one Hibernate Session that is
scoped to the whole request. Use getCurrentSession(), so that
it is automatically bound to the current Java thread.

Next, the possible actions of the request are processed and the response HTML
is rendered. We will get to that part soon.

Finally, the unit of work ends when processing and rendering are complete. If any
problems occurred during processing or rendering, an exception will be thrown
and the database transaction rolled back. This completes the
session-per-request pattern. Instead of the transaction
demarcation code in every servlet, you could also write a servlet filter.
See the Hibernate website and Wiki for more information about this pattern
called Open Session in View. You will need it as soon
as you consider rendering your view in JSP, not in a servlet.

1.3.2. Processing and rendering

Now you can implement the processing of the request and the rendering of the page.

This coding style, with a mix of Java and HTML, would not scale
in a more complex application-keep in mind that we are only illustrating
basic Hibernate concepts in this tutorial. The code prints an HTML
header and a footer. Inside this page, an HTML form for event entry and
a list of all events in the database are printed. The first method is
trivial and only outputs HTML:

The servlet is now complete. A request to the servlet will be processed
in a single Session and Transaction. As
earlier in the standalone application, Hibernate can automatically bind these
objects to the current thread of execution. This gives you the freedom to layer
your code and access the SessionFactory in any way you like.
Usually you would use a more sophisticated design and move the data access code
into data access objects (the DAO pattern). See the Hibernate Wiki for more
examples.

1.3.3. Deploying and testing

To deploy this application for testing we must create a
Web ARchive (WAR). First we must define the WAR descriptor
as src/main/webapp/WEB-INF/web.xml

Note

If you do not have Tomcat installed, download it from
http://tomcat.apache.org/ and follow the
installation instructions. Our application requires
no changes to the standard Tomcat configuration.

Once deployed and Tomcat is running, access the application at
http://localhost:8080/hibernate-tutorial/eventmanager. Make
sure you watch the Tomcat log to see Hibernate initialize when the first
request hits your servlet (the static initializer in HibernateUtil
is called) and to get the detailed output if any exceptions occurs.

1.4. Summary

This tutorial covered the basics of writing a simple standalone Hibernate application
and a small web application. More tutorials are available from the Hibernate
website.

Chapter 2. Architecture

2.1. Overview

The diagram below provides a high-level view of the Hibernate architecture:

We do not have the scope in this document to provide a more detailed view of all the runtime architectures available;
Hibernate is flexible and supports several different approaches. We will, however,
show the two extremes: "minimal" architecture and "comprehensive" architecture.

This next diagram illustrates how Hibernate utilizes database and configuration data to
provide persistence services, and persistent objects, to the application.

The "minimal" architecture has the application
provide its own JDBC connections and manage its own transactions. This approach
uses a minimal subset of Hibernate's APIs:

The "comprehensive" architecture abstracts the application away from the
underlying JDBC/JTA APIs and allows Hibernate to manage the details.

Here are some definitions of the objects depicted in the diagrams:

SessionFactory (org.hibernate.SessionFactory)

A threadsafe, immutable cache of compiled mappings for a single database.
A factory for Session and a client of
ConnectionProvider, SessionFactory can hold an optional (second-level)
cache of data that is reusable between transactions at a
process, or cluster, level.

Session (org.hibernate.Session)

A single-threaded, short-lived object representing a conversation between
the application and the persistent store. It wraps a JDBC connection and is a factory
for Transaction. Session holds a mandatory first-level cache
of persistent objects that are used when navigating the object graph or looking up
objects by identifier.

Persistent objects and collections

Short-lived, single threaded objects containing persistent state and business
function. These can be ordinary JavaBeans/POJOs. They are associated with exactly one
Session. Once the Session is closed,
they will be detached and free to use in any application layer (for example, directly
as data transfer objects to and from presentation).

Transient and detached objects and collections

Instances of persistent classes that are not currently associated with a
Session. They may have been instantiated by
the application and not yet persisted, or they may have been instantiated by a
closed Session.

Transaction (org.hibernate.Transaction)

(Optional) A single-threaded, short-lived object used by the application to
specify atomic units of work. It abstracts the application from the underlying JDBC,
JTA or CORBA transaction. A Session might span several
Transactions in some cases. However, transaction demarcation,
either using the underlying API or Transaction, is never
optional.

ConnectionProvider (org.hibernate.connection.ConnectionProvider)

(Optional) A factory for, and pool of, JDBC connections. It abstracts the application from
underlying Datasource or DriverManager.
It is not exposed to application, but it can be extended and/or implemented by the developer.

TransactionFactory (org.hibernate.TransactionFactory)

(Optional) A factory for Transaction instances. It is not exposed to the
application, but it can be extended and/or implemented by the developer.

Extension Interfaces

Hibernate offers a range of optional extension interfaces you can implement to customize
the behavior of your persistence layer. See the API documentation for details.

Given a "minimal" architecture, the application bypasses the
Transaction/TransactionFactory and/or
ConnectionProvider APIs to communicate with JTA or JDBC directly.

2.2. Instance states

An instance of a persistent class can be in one of three different states. These states are
defined in relation to a persistence context.
The Hibernate Session object is the persistence context. The three different states are as follows:

transient

The instance is not associated with
any persistence context. It has no persistent identity or
primary key value.

persistent

The instance is currently associated with a persistence
context. It has a persistent identity (primary key value)
and can have a corresponding row in the database. For a
particular persistence context, Hibernate
guarantees that persistent identity
is equivalent to Java identity in relation to the in-memory location of the
object.

detached

The instance was once associated with a persistence
context, but that context was closed, or the instance
was serialized to another process. It has a persistent
identity and can have a corresponding row in the database.
For detached instances, Hibernate does not guarantee
the relationship between persistent identity and
Java identity.

2.3. JMX Integration

JMX is the J2EE standard for the management of Java components. Hibernate can be managed via
a JMX standard service. AN MBean implementation is provided in the distribution:
org.hibernate.jmx.HibernateService.

For an example of how to deploy Hibernate as a JMX service on the JBoss Application Server,
please see the JBoss User Guide. JBoss AS also provides these benefits if you deploy
using JMX:

Session Management: the Hibernate Session's life cycle
can be automatically bound to the scope of a JTA transaction. This means that you no
longer have to manually open and close the Session; this
becomes the job of a JBoss EJB interceptor. You also do not have to worry about
transaction demarcation in your code (if you would like to write a portable
persistence layer use the optional Hibernate Transaction
API for this). You call the HibernateContext to access a
Session.

HAR deployment: the Hibernate JMX service is deployed using a JBoss
service deployment descriptor in an EAR and/or SAR file, as it supports all the usual
configuration options of a Hibernate SessionFactory. However, you still
need to name all your mapping files in the deployment descriptor. If you use
the optional HAR deployment, JBoss will automatically detect all mapping files in your
HAR file.

Consult the JBoss AS user guide for more information about these options.

2.4. JCA Support

Hibernate can also be configured as a JCA connector. Please see the website for more
information. Please note, however, that at this stage Hibernate JCA support is under development.

2.5. Contextual sessions

Most applications using Hibernate need some form of "contextual" session, where a given
session is in effect throughout the scope of a given context. However, across applications
the definition of what constitutes a context is typically different; different contexts
define different scopes to the notion of current. Applications using Hibernate prior
to version 3.0 tended to utilize either home-grown ThreadLocal-based
contextual sessions, helper classes such as HibernateUtil, or utilized
third-party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession()
method. Initially, this assumed usage of JTA transactions, where the
JTA transaction defined both the scope and context of a current session.
Given the maturity of the numerous stand-alone
JTA TransactionManager implementations, most, if not all,
applications should be using JTA transaction management, whether or not
they are deployed into a J2EE container. Based on that, the
JTA-based contextual sessions are all you need to use.

However, as of version 3.1, the processing behind
SessionFactory.getCurrentSession() is now pluggable. To that
end, a new extension interface, org.hibernate.context.CurrentSessionContext,
and a new configuration parameter, hibernate.current_session_context_class,
have been added to allow pluggability of the scope and context of defining current sessions.

See the Javadocs for the org.hibernate.context.CurrentSessionContext
interface for a detailed discussion of its contract. It defines a single method,
currentSession(), by which the implementation is responsible for
tracking the current contextual session. Out-of-the-box, Hibernate comes with three
implementations of this interface:

org.hibernate.context.JTASessionContext: current sessions
are tracked and scoped by a JTA transaction. The processing
here is exactly the same as in the older JTA-only approach. See the Javadocs
for details.

org.hibernate.context.ThreadLocalSessionContext:current
sessions are tracked by thread of execution. See the Javadocs for details.

org.hibernate.context.ManagedSessionContext: current
sessions are tracked by thread of execution. However, you are responsible to
bind and unbind a Session instance with static methods
on this class: it does not open, flush, or close a Session.

The first two implementations provide a "one session - one database transaction" programming
model. This is also known and used as session-per-request. The beginning
and end of a Hibernate session is defined by the duration of a database transaction.
If you use programmatic transaction demarcation in plain JSE without JTA, you are advised to
use the Hibernate Transaction API to hide the underlying transaction system
from your code. If you use JTA, you can utilize the JTA interfaces to demarcate transactions. If you
execute in an EJB container that supports CMT, transaction boundaries are defined declaratively
and you do not need any transaction or session demarcation operations in your code.
Refer to Chapter 12, Transactions and Concurrency for more information and code examples.

The hibernate.current_session_context_class configuration parameter
defines which org.hibernate.context.CurrentSessionContext implementation
should be used. For backwards compatibility, if this configuration parameter is not set
but a org.hibernate.transaction.TransactionManagerLookup is configured,
Hibernate will use the org.hibernate.context.JTASessionContext.
Typically, the value of this parameter would just name the implementation class to
use. For the three out-of-the-box implementations, however, there are three corresponding
short names: "jta", "thread", and "managed".

Hibernate is designed to operate in many different environments and, as such, there
is a broad range of configuration parameters. Fortunately, most have sensible
default values and Hibernate is distributed with an example
hibernate.properties file in etc/ that displays
the various options. Simply put the example file in your classpath and customize it to suit your needs.

3.1. Programmatic configuration

An instance of org.hibernate.cfg.Configuration represents an entire set of mappings
of an application's Java types to an SQL database. The org.hibernate.cfg.Configuration
is used to build an immutable org.hibernate.SessionFactory. The mappings
are compiled from various XML mapping files.

You can obtain a org.hibernate.cfg.Configuration instance by instantiating
it directly and specifying XML mapping documents. If the mapping files are in the classpath,
use addResource(). For example:

This is not the only way to pass configuration properties to Hibernate.
Some alternative options include:

Pass an instance of java.util.Properties to
Configuration.setProperties().

Place a file named hibernate.properties in a root directory of the classpath.

Set System properties using java -Dproperty=value.

Include <property> elements in
hibernate.cfg.xml (this is discussed later).

If you want to get started quicklyhibernate.properties is the easiest approach.

The org.hibernate.cfg.Configuration is intended as a startup-time object that will
be discarded once a SessionFactory is created.

3.2. Obtaining a SessionFactory

When all mappings have been parsed by the org.hibernate.cfg.Configuration,
the application must obtain a factory for org.hibernate.Session instances.
This factory is intended to be shared by all application threads:

SessionFactory sessions = cfg.buildSessionFactory();

Hibernate does allow your application to instantiate more than one
org.hibernate.SessionFactory. This is useful if you are using more than
one database.

3.3. JDBC connections

It is advisable to have the org.hibernate.SessionFactory create and pool
JDBC connections for you. If you take this approach, opening a org.hibernate.Session
is as simple as:

Session session = sessions.openSession();// open a newSession

Once you start a task that requires access to the database, a JDBC connection will be obtained from
the pool.

Before you can do this, you first need to pass some JDBC connection properties to Hibernate. All Hibernate property
names and semantics are defined on the class org.hibernate.cfg.Environment.
The most important settings for JDBC connection configuration are outlined below.

Hibernate will obtain and pool connections using java.sql.DriverManager
if you set the following properties:

Table 3.1. Hibernate JDBC Properties

Property name

Purpose

hibernate.connection.driver_class

JDBC driver class

hibernate.connection.url

JDBC URL

hibernate.connection.username

database user

hibernate.connection.password

database user password

hibernate.connection.pool_size

maximum number of pooled connections

Hibernate's own connection pooling algorithm is, however, quite rudimentary.
It is intended to help you get started and is not intended for use
in a production system, or even for performance testing. You should
use a third party pool for best performance and stability. Just replace the
hibernate.connection.pool_size property with connection
pool specific settings. This will turn off Hibernate's internal pool. For
example, you might like to use c3p0.

C3P0 is an open source JDBC connection pool distributed along with Hibernate in the lib
directory. Hibernate will use its org.hibernate.connection.C3P0ConnectionProvider
for connection pooling if you set hibernate.c3p0.* properties. If you would like to use Proxool,
refer to the packaged hibernate.properties and the Hibernate web site for more
information.

For use inside an application server, you should almost always configure Hibernate to obtain connections
from an application server javax.sql.Datasource registered in JNDI. You will
need to set at least one of the following properties:

Table 3.2. Hibernate Datasource Properties

Property name

Purpose

hibernate.connection.datasource

datasource JNDI name

hibernate.jndi.url

URL of the JNDI provider (optional)

hibernate.jndi.class

class of the JNDI InitialContextFactory (optional)

hibernate.connection.username

database user (optional)

hibernate.connection.password

database user password (optional)

Here is an example hibernate.properties file for an application server provided JNDI
datasource:

JDBC connections obtained from a JNDI datasource will automatically participate
in the container-managed transactions of the application server.

Arbitrary connection properties can be given by prepending "hibernate.connection" to the
connection property name. For example, you can specify a charSet
connection property using hibernate.connection.charSet.

You can define your own plugin strategy for obtaining JDBC connections by implementing the
interface org.hibernate.connection.ConnectionProvider, and specifying your
custom implementation via the hibernate.connection.provider_class property.

3.4. Optional configuration properties

There are a number of other properties that control the behavior of Hibernate at runtime. All are optional
and have reasonable default values.

Warning

Some of these properties are "system-level" only. System-level properties can
be set only via java -Dproperty=value or hibernate.properties. They
cannot be set by the other techniques described above.

Table 3.3. Hibernate Configuration Properties

Property name

Purpose

hibernate.dialect

The classname of a Hibernate org.hibernate.dialect.Dialect which
allows Hibernate to generate SQL optimized for a particular relational database.

e.g.full.classname.of.Dialect

In most cases Hibernate will actually be able to choose the correct
org.hibernate.dialect.Dialect implementation based on the
JDBC metadata returned by the JDBC driver.

hibernate.show_sql

Write all SQL statements to console. This is an alternative
to setting the log category org.hibernate.SQL
to debug.

e.g.true | false

hibernate.format_sql

Pretty print the SQL in the log and console.

e.g.true | false

hibernate.default_schema

Qualify unqualified table names with the given schema/tablespace
in generated SQL.

e.g.SCHEMA_NAME

hibernate.default_catalog

Qualifies unqualified table names with the given catalog
in generated SQL.

e.g.CATALOG_NAME

hibernate.session_factory_name

The org.hibernate.SessionFactory will be automatically
bound to this name in JNDI after it has been created.

Set this property to true if your JDBC driver returns
correct row counts from executeBatch(). It is usually
safe to turn this option on. Hibernate will then use batched DML for
automatically versioned data. Defaults to false.

e.g.true | false

hibernate.jdbc.factory_class

Select a custom org.hibernate.jdbc.Batcher. Most applications
will not need this configuration property.

e.g.classname.of.BatcherFactory

hibernate.jdbc.use_scrollable_resultset

Enables use of JDBC2 scrollable resultsets by Hibernate.
This property is only necessary when using user-supplied
JDBC connections. Hibernate uses connection metadata otherwise.

Enables use of JDBC3 PreparedStatement.getGeneratedKeys()
to retrieve natively generated keys after insert. Requires JDBC3+ driver
and JRE1.4+, set to false if your driver has problems with the Hibernate
identifier generators. By default, it tries to determine the driver capabilities
using connection metadata.

e.g.true|false

hibernate.connection.provider_class

The classname of a custom org.hibernate.connection.ConnectionProvider
which provides JDBC connections to Hibernate.

e.g.classname.of.ConnectionProvider

hibernate.connection.isolation

Sets the JDBC transaction isolation level. Check java.sql.Connection
for meaningful values, but note that most databases do not support all isolation levels and some
define additional, non-standard isolations.

e.g.1, 2, 4, 8

hibernate.connection.autocommit

Enables autocommit for JDBC pooled connections (it is not recommended).

e.g.true | false

hibernate.connection.release_mode

Specifies when Hibernate should release JDBC connections. By default,
a JDBC connection is held until the session is explicitly closed or
disconnected. For an application server JTA datasource, use
after_statement to aggressively release connections
after every JDBC call. For a non-JTA connection, it often makes sense to
release the connection at the end of each transaction, by using
after_transaction. auto will
choose after_statement for the JTA and CMT transaction
strategies and after_transaction for the JDBC
transaction strategy.

e.g.auto (default) | on_close |
after_transaction | after_statement

This setting only affects Sessions returned from
SessionFactory.openSession. For Sessions
obtained through SessionFactory.getCurrentSession, the
CurrentSessionContext implementation configured for use
controls the connection release mode for those Sessions.
See Section 2.5, “Contextual sessions”

Optimizes second-level cache operation to minimize writes, at the
cost of more frequent reads. This setting is most useful for
clustered caches and, in Hibernate3, is enabled by default for
clustered cache implementations.

e.g.true|false

hibernate.cache.use_query_cache

Enables the query cache. Individual queries still have to be set cachable.

e.g.true|false

hibernate.cache.use_second_level_cache

Can be used to completely disable the second level cache, which is enabled
by default for classes which specify a <cache>
mapping.

e.g.true|false

hibernate.cache.query_cache_factory

The classname of a custom QueryCache interface,
defaults to the built-in StandardQueryCache.

e.g.classname.of.QueryCache

hibernate.cache.region_prefix

A prefix to use for second-level cache region names.

e.g.prefix

hibernate.cache.use_structured_entries

Forces Hibernate to store data in the second-level cache
in a more human-friendly format.

e.g.true|false

Table 3.6. Hibernate Transaction Properties

Property name

Purpose

hibernate.transaction.factory_class

The classname of a TransactionFactory
to use with Hibernate Transaction API
(defaults to JDBCTransactionFactory).

e.g.classname.of.TransactionFactory

jta.UserTransaction

A JNDI name used by JTATransactionFactory to
obtain the JTA UserTransaction from the
application server.

e.g.jndi/composite/name

hibernate.transaction.manager_lookup_class

The classname of a TransactionManagerLookup. It is
required when JVM-level caching is enabled or when using hilo
generator in a JTA environment.

e.g.classname.of.TransactionManagerLookup

hibernate.transaction.flush_before_completion

If enabled, the session will be automatically flushed during the
before completion phase of the transaction. Built-in and
automatic session context management is preferred, see
Section 2.5, “Contextual sessions”.

e.g.true | false

hibernate.transaction.auto_close_session

If enabled, the session will be automatically closed during the
after completion phase of the transaction. Built-in and
automatic session context management is preferred, see
Section 2.5, “Contextual sessions”.

e.g.org.hibernate.hql.ast.ASTQueryTranslatorFactory or
org.hibernate.hql.classic.ClassicQueryTranslatorFactory

hibernate.query.substitutions

Is used to map from tokens in Hibernate queries to SQL tokens
(tokens might be function or literal names, for example).

e.g.hqlLiteral=SQL_LITERAL, hqlFunction=SQLFUNC

hibernate.hbm2ddl.auto

Automatically validates or exports schema DDL to the database
when the SessionFactory is created. With
create-drop, the database schema will be
dropped when the SessionFactory is closed
explicitly.

e.g.validate | update |
create | create-drop

hibernate.bytecode.use_reflection_optimizer

Enables the use of bytecode manipulation instead of
runtime reflection. This is a System-level property and cannot be set
in hibernate.cfg.xml.
Reflection can sometimes be useful when troubleshooting.
Hibernate always requires either CGLIB or javassist even
if you turn off the optimizer.

e.g.true |
false

hibernate.bytecode.provider

Both javassist or cglib can be used as byte manipulation
engines; the default is javassist.

e.g.javassist |
cglib

3.4.1. SQL Dialects

Always set the hibernate.dialect property to the correct
org.hibernate.dialect.Dialect subclass for your database. If you
specify a dialect, Hibernate will use sensible defaults for some of the
other properties listed above. This means that you will not have to specify them manually.

Table 3.8. Hibernate SQL Dialects (hibernate.dialect)

RDBMS

Dialect

DB2

org.hibernate.dialect.DB2Dialect

DB2 AS/400

org.hibernate.dialect.DB2400Dialect

DB2 OS390

org.hibernate.dialect.DB2390Dialect

PostgreSQL

org.hibernate.dialect.PostgreSQLDialect

MySQL

org.hibernate.dialect.MySQLDialect

MySQL with InnoDB

org.hibernate.dialect.MySQLInnoDBDialect

MySQL with MyISAM

org.hibernate.dialect.MySQLMyISAMDialect

Oracle (any version)

org.hibernate.dialect.OracleDialect

Oracle 9i

org.hibernate.dialect.Oracle9iDialect

Oracle 10g

org.hibernate.dialect.Oracle10gDialect

Sybase

org.hibernate.dialect.SybaseDialect

Sybase Anywhere

org.hibernate.dialect.SybaseAnywhereDialect

Microsoft SQL Server

org.hibernate.dialect.SQLServerDialect

SAP DB

org.hibernate.dialect.SAPDBDialect

Informix

org.hibernate.dialect.InformixDialect

HypersonicSQL

org.hibernate.dialect.HSQLDialect

Ingres

org.hibernate.dialect.IngresDialect

Progress

org.hibernate.dialect.ProgressDialect

Mckoi SQL

org.hibernate.dialect.MckoiDialect

Interbase

org.hibernate.dialect.InterbaseDialect

Pointbase

org.hibernate.dialect.PointbaseDialect

FrontBase

org.hibernate.dialect.FrontbaseDialect

Firebird

org.hibernate.dialect.FirebirdDialect

3.4.2. Outer Join Fetching

If your database supports ANSI, Oracle or Sybase style outer joins, outer join
fetching will often increase performance by limiting the number of round
trips to and from the database. This is, however, at the cost of possibly more work performed by
the database itself. Outer join fetching allows a whole graph of objects connected
by many-to-one, one-to-many, many-to-many and one-to-one associations to be retrieved
in a single SQL SELECT.

Outer join fetching can be disabled globally by setting
the property hibernate.max_fetch_depth to 0.
A setting of 1 or higher enables outer join fetching for
one-to-one and many-to-one associations that have been mapped with
fetch="join".

3.4.3. Binary Streams

Oracle limits the size of byte arrays that can
be passed to and/or from its JDBC driver. If you wish to use large instances of
binary or serializable type, you should
enable hibernate.jdbc.use_streams_for_binary.
This is a system-level setting only.

3.4.4. Second-level and query cache

The properties prefixed by hibernate.cache
allow you to use a process or cluster scoped second-level cache system
with Hibernate. See the Section 20.2, “The Second Level Cache” for
more information.

3.4.5. Query Language Substitution

You can define new Hibernate query tokens using hibernate.query.substitutions.
For example:

hibernate.query.substitutions true=1, false=0

This would cause the tokens true and false to be translated to
integer literals in the generated SQL.

hibernate.query.substitutions toLowercase=LOWER

This would allow you to rename the SQL LOWER function.

3.4.6. Hibernate statistics

If you enable hibernate.generate_statistics, Hibernate
exposes a number of metrics that are useful when tuning a running system via
SessionFactory.getStatistics(). Hibernate can even be configured
to expose these statistics via JMX. Read the Javadoc of the interfaces in
org.hibernate.stats for more information.

3.5. Logging

Hibernate utilizes Simple Logging Facade for Java
(SLF4J) in order to log various system events. SLF4J can direct your logging output to
several logging frameworks (NOP, Simple, log4j version 1.2, JDK 1.4 logging, JCL or logback) depending on your
chosen binding. In order to setup logging you will need slf4j-api.jar in
your classpath together with the jar file for your preferred binding - slf4j-log4j12.jar
in the case of Log4J. See the SLF4J documentation for more detail.
To use Log4j you will also need to place a log4j.properties file in your classpath.
An example properties file is distributed with Hibernate in the src/ directory.

It is recommended that you familiarize yourself with Hibernate's log
messages. A lot of work has been put into making the Hibernate log as
detailed as possible, without making it unreadable. It is an essential
troubleshooting device. The most interesting log categories are the
following:

Table 3.9. Hibernate Log Categories

Category

Function

org.hibernate.SQL

Log all SQL DML statements as they are executed

org.hibernate.type

Log all JDBC parameters

org.hibernate.tool.hbm2ddl

Log all SQL DDL statements as they are executed

org.hibernate.pretty

Log the state of all entities (max 20 entities) associated
with the session at flush time

org.hibernate.cache

Log all second-level cache activity

org.hibernate.transaction

Log transaction related activity

org.hibernate.jdbc

Log all JDBC resource acquisition

org.hibernate.hql.ast.AST

Log HQL and SQL ASTs during query parsing

org.hibernate.secure

Log all JAAS authorization requests

org.hibernate

Log everything. This is a lot of information but it is useful for
troubleshooting

When developing applications with Hibernate, you should almost always work with
debug enabled for the category org.hibernate.SQL,
or, alternatively, the property hibernate.show_sql enabled.

3.6. Implementing a NamingStrategy

The interface org.hibernate.cfg.NamingStrategy allows you
to specify a "naming standard" for database objects and schema elements.

You can provide rules for automatically generating database identifiers from
Java identifiers or for processing "logical" column and table names given in
the mapping file into "physical" table and column names. This feature helps
reduce the verbosity of the mapping document, eliminating repetitive noise
(TBL_ prefixes, for example). The default strategy used by
Hibernate is quite minimal.

You can specify a different strategy by calling
Configuration.setNamingStrategy() before adding mappings:

org.hibernate.cfg.ImprovedNamingStrategy is a built-in
strategy that might be a useful starting point for some applications.

3.7. XML configuration file

An alternative approach to configuration is to specify a full configuration in
a file named hibernate.cfg.xml. This file can be used as a
replacement for the hibernate.properties file or, if both
are present, to override properties.

The XML configuration file is by default expected to be in the root of
your CLASSPATH. Here is an example:

The advantage of this approach is the externalization of the
mapping file names to configuration. The hibernate.cfg.xml
is also more convenient once you have to tune the Hibernate cache. It is
your choice to use either hibernate.properties or
hibernate.cfg.xml. Both are equivalent, except for the above
mentioned benefits of using the XML syntax.

3.8. J2EE Application Server integration

Hibernate has the following integration points for J2EE infrastructure:

Container-managed datasources: Hibernate can use
JDBC connections managed by the container and provided through JNDI. Usually,
a JTA compatible TransactionManager and a
ResourceManager take care of transaction management (CMT),
especially distributed transaction handling across several datasources. You can
also demarcate transaction boundaries programmatically (BMT), or
you might want to use the optional Hibernate Transaction
API for this to keep your code portable.

Automatic JNDI binding: Hibernate can bind its
SessionFactory to JNDI after startup.

JTA Session binding: the Hibernate Session
can be automatically bound to the scope of JTA transactions. Simply
lookup the SessionFactory from JNDI and get the current
Session. Let Hibernate manage flushing and closing the
Session when your JTA transaction completes. Transaction
demarcation is either declarative (CMT) or programmatic (BMT/UserTransaction).

JMX deployment: if you have a JMX capable application server
(e.g. JBoss AS), you can choose to deploy Hibernate as a managed MBean. This saves
you the one line startup code to build your SessionFactory from
a Configuration. The container will startup your
HibernateService and also take care of service
dependencies (datasource has to be available before Hibernate starts, etc).

Depending on your environment, you might have to set the configuration option
hibernate.connection.aggressive_release to true if your
application server shows "connection containment" exceptions.

3.8.1. Transaction strategy configuration

The Hibernate Session API is independent of any transaction
demarcation system in your architecture. If you let Hibernate use JDBC directly
through a connection pool, you can begin and end your transactions by calling
the JDBC API. If you run in a J2EE application server, you might want to use bean-managed
transactions and call the JTA API and UserTransaction when needed.

To keep your code portable between these two (and other) environments we recommend the optional
Hibernate Transaction API, which wraps and hides the underlying system.
You have to specify a factory class for Transaction instances by setting the
Hibernate configuration property hibernate.transaction.factory_class.

There are three standard, or built-in, choices:

org.hibernate.transaction.JDBCTransactionFactory

delegates to database (JDBC) transactions (default)

org.hibernate.transaction.JTATransactionFactory

delegates to container-managed transactions if an existing transaction is
underway in this context (for example, EJB session bean method). Otherwise,
a new transaction is started and bean-managed transactions are used.

org.hibernate.transaction.CMTTransactionFactory

delegates to container-managed JTA transactions

You can also define your own transaction strategies (for a CORBA transaction service,
for example).

Some features in Hibernate (i.e., the second level cache, Contextual Sessions with JTA, etc.)
require access to the JTA TransactionManager in a managed environment.
In an application server, since J2EE does not standardize a single mechanism, you have to specify how Hibernate should obtain a reference to the
TransactionManager:

Table 3.10. JTA TransactionManagers

Transaction Factory

Application Server

org.hibernate.transaction.JBossTransactionManagerLookup

JBoss

org.hibernate.transaction.WeblogicTransactionManagerLookup

Weblogic

org.hibernate.transaction.WebSphereTransactionManagerLookup

WebSphere

org.hibernate.transaction.WebSphereExtendedJTATransactionLookup

WebSphere 6

org.hibernate.transaction.OrionTransactionManagerLookup

Orion

org.hibernate.transaction.ResinTransactionManagerLookup

Resin

org.hibernate.transaction.JOTMTransactionManagerLookup

JOTM

org.hibernate.transaction.JOnASTransactionManagerLookup

JOnAS

org.hibernate.transaction.JRun4TransactionManagerLookup

JRun4

org.hibernate.transaction.BESTransactionManagerLookup

Borland ES

3.8.2. JNDI-bound SessionFactory

A JNDI-bound Hibernate SessionFactory can simplify the lookup
function of the factory and create new Sessions. This
is not, however, related to a JNDI bound Datasource; both simply use the
same registry.

If you wish to have the SessionFactory bound to a JNDI namespace, specify
a name (e.g. java:hibernate/SessionFactory) using the property
hibernate.session_factory_name. If this property is omitted, the
SessionFactory will not be bound to JNDI. This is especially useful in
environments with a read-only JNDI default implementation (in Tomcat, for example).

When binding the SessionFactory to JNDI, Hibernate will use the values of
hibernate.jndi.url, hibernate.jndi.class to instantiate
an initial context. If they are not specified, the default InitialContext
will be used.

Hibernate will automatically place the SessionFactory in JNDI after
you call cfg.buildSessionFactory(). This means you will have
this call in some startup code, or utility class in your application, unless you use
JMX deployment with the HibernateService (this is discussed later in greater detail).

If you use a JNDI SessionFactory, an EJB or any other class, you can
obtain the SessionFactory using a JNDI lookup.

It is recommended that you bind the SessionFactory to JNDI in
a managed environment and use a static singleton otherwise.
To shield your application code from these details, we also recommend to hide the
actual lookup code for a SessionFactory in a helper class,
such as HibernateUtil.getSessionFactory(). Note that such a
class is also a convenient way to startup Hibernate—see chapter 1.

3.8.3. Current Session context management with JTA

The easiest way to handle Sessions and transactions is
Hibernate's automatic "current" Session management.
For a discussion of contextual sessions see Section 2.5, “Contextual sessions”.
Using the "jta" session context, if there is no Hibernate
Session associated with the current JTA transaction, one will
be started and associated with that JTA transaction the first time you call
sessionFactory.getCurrentSession(). The Sessions
retrieved via getCurrentSession() in the "jta" context
are set to automatically flush before the transaction completes, close
after the transaction completes, and aggressively release JDBC connections
after each statement. This allows the Sessions to
be managed by the life cycle of the JTA transaction to which it is associated,
keeping user code clean of such management concerns. Your code can either use
JTA programmatically through UserTransaction, or (recommended
for portable code) use the Hibernate Transaction API to set
transaction boundaries. If you run in an EJB container, declarative transaction
demarcation with CMT is preferred.

3.8.4. JMX deployment

The line cfg.buildSessionFactory() still has to be executed
somewhere to get a SessionFactory into JNDI. You can do this
either in a static initializer block, like the one in
HibernateUtil, or you can deploy Hibernate as a managed
service.

Hibernate is distributed with org.hibernate.jmx.HibernateService
for deployment on an application server with JMX capabilities, such as JBoss AS.
The actual deployment and configuration is vendor-specific. Here is an example
jboss-service.xml for JBoss 4.0.x:

This file is deployed in a directory called META-INF and packaged
in a JAR file with the extension .sar (service archive). You also need
to package Hibernate, its required third-party libraries, your compiled persistent classes,
as well as your mapping files in the same archive. Your enterprise beans (usually session
beans) can be kept in their own JAR file, but you can include this EJB JAR file in the
main service archive to get a single (hot-)deployable unit. Consult the JBoss AS
documentation for more information about JMX service and EJB deployment.

Persistent classes are classes in an application that implement the entities
of the business problem (e.g. Customer and Order in an E-commerce application).
Not all instances of a persistent class are considered to be in the persistent
state. For example, an instance can instead be transient or detached.

Hibernate works best if these classes follow some simple rules, also known
as the Plain Old Java Object (POJO) programming model. However, none of these
rules are hard requirements. Indeed, Hibernate3 assumes very little about
the nature of your persistent objects. You can express a domain model in other
ways (using trees of Map instances, for example).

The four main rules of persistent classes are explored in more detail in the following sections.

4.1.1. Implement a no-argument constructor

Cat has a no-argument constructor. All persistent classes must
have a default constructor (which can be non-public) so that Hibernate can instantiate
them using Constructor.newInstance(). It is recommended that you have a
default constructor with at least package visibility for runtime proxy
generation in Hibernate.

4.1.2. Provide an identifier property (optional)

Cat has a property called id. This property
maps to the primary key column of a database table. The property might have been called
anything, and its type might have been any primitive type, any primitive "wrapper"
type, java.lang.String or java.util.Date. If
your legacy database table has composite keys, you can use a user-defined class
with properties of these types (see the section on composite identifiers later in the chapter.)

The identifier property is strictly optional. You can leave them off and let Hibernate
keep track of object identifiers internally. We do not recommend this, however.

In fact, some functionality is available only to classes that declare an
identifier property:

We recommend that you declare consistently-named identifier properties on persistent
classes and that you use a nullable (i.e., non-primitive) type.

4.1.3. Prefer non-final classes (optional)

A central feature of Hibernate, proxies, depends upon the
persistent class being either non-final, or the implementation of an interface
that declares all public methods.

You can persist final classes that do not implement an interface
with Hibernate. You will not, however, be able to use proxies for lazy association fetching which
will ultimately limit your options for performance tuning.

You should also avoid declaring public final methods on the
non-final classes. If you want to use a class with a public final
method, you must explicitly disable proxying by setting lazy="false".

Cat declares accessor methods for all its persistent fields.
Many other ORM tools directly persist instance variables. It is
better to provide an indirection between the relational schema and internal
data structures of the class. By default, Hibernate persists JavaBeans style
properties and recognizes method names of the form getFoo,
isFoo and setFoo. If required, you can switch to direct
field access for particular properties.

Properties need not be declared public - Hibernate can
persist a property with a default, protected or
private get / set pair.

4.2. Implementing inheritance

A subclass must also observe the first and second rules. It inherits its
identifier property from the superclass, Cat. For example:

4.3. Implementing equals() and hashCode()

intend to put instances of persistent classes in a Set
(the recommended way to represent many-valued associations);
and

intend to use reattachment of detached instances

Hibernate guarantees equivalence of persistent identity (database row) and Java identity
only inside a particular session scope. When you mix instances retrieved in
different sessions, you must implement equals() and
hashCode() if you wish to have meaningful semantics for
Sets.

The most obvious way is to implement equals()/hashCode()
by comparing the identifier value of both objects. If the value is the same, both must
be the same database row, because they are equal. If both are added to a Set,
you will only have one element in the Set). Unfortunately, you cannot use that
approach with generated identifiers. Hibernate will only assign identifier values to objects
that are persistent; a newly created instance will not have any identifier value. Furthermore,
if an instance is unsaved and currently in a Set, saving it will assign
an identifier value to the object. If equals() and hashCode()
are based on the identifier value, the hash code would change, breaking the contract of the
Set. See the Hibernate website for a full discussion of this problem. This is not
a Hibernate issue, but normal Java semantics of object identity and equality.

It is recommended that you implement equals() and hashCode()
using Business key equality. Business key equality means that the
equals() method compares only the properties that form the business
key. It is a key that would identify our instance in the real world (a
natural candidate key):

4.4. Dynamic models

Note

The following features are currently considered
experimental and may change in the near future.

Persistent entities do not necessarily have to be represented as POJO classes
or as JavaBean objects at runtime. Hibernate also supports dynamic models
(using Maps of Maps at runtime) and the
representation of entities as DOM4J trees. With this approach, you do not
write persistent classes, only mapping files.

By default, Hibernate works in normal POJO mode. You can set a default entity
representation mode for a particular SessionFactory using the
default_entity_mode configuration option (see
Table 3.3, “Hibernate Configuration Properties”).

The following examples demonstrate the representation using Maps.
First, in the mapping file an entity-name has to be declared
instead of, or in addition to, a class name:

One of the main advantages of dynamic mapping is quick turnaround time for prototyping,
without the need for entity class implementation. However, you lose compile-time
type checking and will likely deal with many exceptions at runtime. As a result of
the Hibernate mapping, the database schema can easily be normalized and sound,
allowing to add a proper domain model implementation on top later on.

Please note that the call to getSession() using an
EntityMode is on the Session API, not the
SessionFactory. That way, the new Session
shares the underlying JDBC connection, transaction, and other context
information. This means you do not have to call flush()
and close() on the secondary Session, and
also leave the transaction and connection handling to the primary unit of work.

4.5. Tuplizers

org.hibernate.tuple.Tuplizer, and its sub-interfaces, are responsible
for managing a particular representation of a piece of data given that representation's
org.hibernate.EntityMode. If a given piece of data is thought of as
a data structure, then a tuplizer is the thing that knows how to create such a data structure
and how to extract values from and inject values into such a data structure. For example,
for the POJO entity mode, the corresponding tuplizer knows how create the POJO through its
constructor. It also knows how to access the POJO properties using the defined property accessors.

There are two high-level types of Tuplizers, represented by the
org.hibernate.tuple.entity.EntityTuplizer and org.hibernate.tuple.component.ComponentTuplizer
interfaces. EntityTuplizers are responsible for managing the above mentioned
contracts in regards to entities, while ComponentTuplizers do the same for
components.

Users can also plug in their own tuplizers. Perhaps you require that a java.util.Map
implementation other than java.util.HashMap be used while in the
dynamic-map entity-mode. Or perhaps you need to define a different proxy generation strategy
than the one used by default. Both would be achieved by defining a custom tuplizer
implementation. Tuplizer definitions are attached to the entity or component mapping they
are meant to manage. Going back to the example of our customer entity:

4.6. EntityNameResolvers

The org.hibernate.EntityNameResolver interface is a contract for resolving the
entity name of a given entity instance. The interface defines a single method resolveEntityName
which is passed the entity instance and is expected to return the appropriate entity name (null is allowed and
would indicate that the resolver does not know how to resolve the entity name of the given entity instance).
Generally speaking, an org.hibernate.EntityNameResolver is going to be most
useful in the case of dynamic models. One example might be using proxied interfaces as your domain model. The
hibernate test suite has an example of this exact style of usage under the
org.hibernate.test.dynamicentity.tuplizer2. Here is some of the code from that package
for illustration.

5.1. Mapping declaration

Object/relational mappings are usually defined in an XML document. The mapping
document is designed to be readable and hand-editable. The mapping language is
Java-centric, meaning that mappings are constructed around persistent class
declarations and not table declarations.

Please note that even though many Hibernate users choose to write the XML by hand,
a number of tools exist to generate the mapping document. These include XDoclet,
Middlegen and AndroMDA.

We will now discuss the content of the mapping document. We will only describe, however, the
document elements and attributes that are used by Hibernate at runtime. The mapping
document also contains some extra optional attributes and elements that affect the
database schemas exported by the schema export tool (for example, the
not-null attribute).

5.1.1. Doctype

All XML mappings should declare the doctype shown. The actual DTD can be found
at the URL above, in the directory hibernate-x.x.x/src/org/hibernate
, or in hibernate3.jar. Hibernate will always look for
the DTD in its classpath first. If you experience lookups of the DTD using an
Internet connection, check the DTD declaration against the contents of your
classpath.

5.1.1.1. EntityResolver

Hibernate will first attempt to resolve DTDs in its classpath.
It does this is by registering a custom org.xml.sax.EntityResolver
implementation with the SAXReader it uses to read in the xml files. This custom
EntityResolver recognizes two different systemId namespaces:

a hibernate namespace is recognized whenever the
resolver encounters a systemId starting with
http://hibernate.sourceforge.net/. The resolver
attempts to resolve these entities via the classloader which loaded
the Hibernate classes.

a user namespace is recognized whenever the
resolver encounters a systemId using a classpath://
URL protocol. The resolver will attempt to resolve these entities
via (1) the current thread context classloader and (2) the
classloader which loaded the Hibernate classes.

Where types.xml is a resource in the your.domain
package and contains a custom typedef.

5.1.2. Hibernate-mapping

This element has several optional attributes. The schema and
catalog attributes specify that tables referred to in this mapping
belong to the named schema and/or catalog. If they are specified, tablenames will be qualified
by the given schema and catalog names. If they are missing, tablenames will be unqualified.
The default-cascade attribute specifies what cascade style
should be assumed for properties and collections that do not specify a
cascade attribute. By default, the auto-import attribute allows you
to use unqualified class names in the query language.

auto-import (optional - defaults to true):
specifies whether we can use unqualified class names of classes in this mapping
in the query language.

package (optional): specifies a package prefix to use for
unqualified class names in the mapping document.

If you have two persistent classes with the same unqualified name, you should set
auto-import="false". An exception will result if you attempt
to assign two classes to the same "imported" name.

The hibernate-mapping element allows you to nest
several persistent <class> mappings, as shown above.
It is, however, good practice (and expected by some tools) to map only a single
persistent class, or a single class hierarchy, in one mapping file and name
it after the persistent superclass. For example, Cat.hbm.xml,
Dog.hbm.xml, or if using inheritance,
Animal.hbm.xml.

5.1.3. Class

You can declare a persistent class using the class element. For example:

name (optional): the fully qualified Java class name of the
persistent class or interface. If this attribute is missing, it is assumed
that the mapping is for a non-POJO entity.

table (optional - defaults to the unqualified class name): the
name of its database table.

discriminator-value (optional - defaults to the class name): a value
that distinguishes individual subclasses that is used for polymorphic behavior. Acceptable
values include null and not null.

mutable (optional - defaults to true): specifies
that instances of the class are (not) mutable.

schema (optional): overrides the schema name specified by
the root <hibernate-mapping> element.

catalog (optional): overrides the catalog name specified by
the root <hibernate-mapping> element.

proxy (optional): specifies an interface to use for lazy
initializing proxies. You can specify the name of the class itself.

dynamic-update (optional - defaults to false):
specifies that UPDATE SQL should be generated at runtime and
can contain only those columns whose values have changed.

dynamic-insert (optional - defaults to false):
specifies that INSERT SQL should be generated at runtime and
contain only the columns whose values are not null.

select-before-update (optional - defaults to false):
specifies that Hibernate should never perform an SQL UPDATE
unless it is certain that an object is actually modified. Only
when a transient object has been associated with a new session using update(),
will Hibernate perform an extra SQL SELECT to determine
if an UPDATE is actually required.

lazy (optional): lazy fetching can be disabled by setting
lazy="false".

(17)

entity-name (optional - defaults to the class name): Hibernate3
allows a class to be mapped multiple times, potentially to different tables.
It also allows entity mappings that are represented by Maps or XML at the Java level.
In these cases, you should provide an explicit arbitrary name for the entity. See
Section 4.4, “Dynamic models” and Chapter 19, XML Mapping
for more information.

(18)

check (optional): an SQL expression used to generate a multi-row
check constraint for automatic schema generation.

(19)

rowid (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use the rowid extra
column for fast updates once this option has been set to rowid. A ROWID
is an implementation detail and represents the physical location of a stored tuple.

(20)

subselect (optional): maps an immutable and read-only entity
to a database subselect. This is useful if you want to have a view instead of a base table.
See below for more information.

(21)

abstract (optional): is used to mark abstract superclasses in
<union-subclass> hierarchies.

It is acceptable for the named persistent class to be an interface. You can
declare implementing classes of that interface using the <subclass>
element. You can persist any static inner class. Specify the
class name using the standard form i.e. e.g.Foo$Bar.

Immutable classes, mutable="false", cannot be updated or deleted by the
application. This allows Hibernate to make some minor performance optimizations.

The optional proxy attribute enables lazy initialization of persistent
instances of the class. Hibernate will initially return CGLIB proxies that implement
the named interface. The persistent object will load when a method of the
proxy is invoked. See "Initializing collections and proxies" below.

Implicit polymorphism means that instances of the class will be returned
by a query that names any superclass or implemented interface or class, and that instances
of any subclass of the class will be returned by a query that names the class itself.
Explicit polymorphism means that class instances will be returned only
by queries that explicitly name that class. Queries that name the class will return
only instances of subclasses mapped inside this <class> declaration
as a <subclass> or <joined-subclass>. For
most purposes, the default polymorphism="implicit" is appropriate.
Explicit polymorphism is useful when two different classes are mapped to the same table
This allows a "lightweight" class that contains a subset of the table columns.

The persister attribute lets you customize the persistence strategy used for
the class. You can, for example, specify your own subclass of
org.hibernate.persister.EntityPersister, or you can even provide a
completely new implementation of the interface
org.hibernate.persister.ClassPersister that implements, for example, persistence via
stored procedure calls, serialization to flat files or LDAP. See
org.hibernate.test.CustomPersister for a simple example of "persistence"
to a Hashtable.

The dynamic-update and dynamic-insert
settings are not inherited by subclasses, so they can also be specified on the
<subclass> or <joined-subclass> elements.
Although these settings can increase performance in some cases, they can actually decrease
performance in others.

Use of select-before-update will usually decrease performance. It is
useful to prevent a database update trigger being called unnecessarily if you reattach a
graph of detached instances to a Session.

If you enable dynamic-update, you will have a choice of optimistic
locking strategies:

version: check the version/timestamp columns

all: check all columns

dirty: check the changed columns, allowing some concurrent updates

none: do not use optimistic locking

It is strongly recommended that you use version/timestamp
columns for optimistic locking with Hibernate.
This strategy optimizes performance and correctly handles modifications
made to detached instances (i.e. when Session.merge() is used).

There is no difference between a view and a base table for a Hibernate mapping.
This is transparent at the database level, although some DBMS do not support
views properly, especially with updates. Sometimes you want to use a view, but you cannot
create one in the database (i.e. with a legacy schema). In this case, you can map an
immutable and read-only entity to a given SQL subselect expression:

Declare the tables to synchronize this entity with, ensuring that auto-flush happens
correctly and that queries against the derived entity do not return stale data.
The <subselect> is available both as an attribute and
a nested mapping element.

5.1.4. id

Mapped classes must declare the primary key column of the database
table. Most classes will also have a JavaBeans-style property holding the unique identifier
of an instance. The <id> element defines the mapping from that
property to the primary key column.

column (optional - defaults to the property name): the
name of the primary key column.

unsaved-value (optional - defaults to a "sensible" value):
an identifier property value that indicates an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or loaded
in a previous session.

access (optional - defaults to property): the
strategy Hibernate should use for accessing the property value.

If the name attribute is missing, it is assumed that the class has no
identifier property.

The unsaved-value attribute is almost never needed in Hibernate3.

There is an alternative <composite-id> declaration that allows access to
legacy data with composite keys. Its use is strongly discouraged for anything else.

5.1.4.1. Generator

The optional <generator> child element names a Java class used
to generate unique identifiers for instances of the persistent class. If any parameters
are required to configure or initialize the generator instance, they are passed using the
<param> element.

All generators implement the interface org.hibernate.id.IdentifierGenerator.
This is a very simple interface. Some applications can choose to provide their own specialized
implementations, however, Hibernate provides a range of built-in implementations. The shortcut
names for the built-in generators are as follows:

increment

generates identifiers of type long, short or
int that are unique only when no other process is inserting data
into the same table.
Do not use in a cluster.

identity

supports identity columns in DB2, MySQL, MS SQL Server, Sybase and
HypersonicSQL. The returned identifier is of type long,
short or int.

sequence

uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator
in Interbase. The returned identifier is of type long,
short or int

hilo

uses a hi/lo algorithm to efficiently generate identifiers of
type long, short or int,
given a table and column (by default hibernate_unique_key and
next_hi respectively) as a source of hi values. The hi/lo
algorithm generates identifiers that are unique only for a particular database.

seqhilo

uses a hi/lo algorithm to efficiently generate identifiers of type
long, short or int,
given a named database sequence.

uuid

uses a 128-bit UUID algorithm to generate identifiers of type string that are
unique within a network (the IP address is used). The UUID is encoded
as a string of 32 hexadecimal digits in length.

guid

uses a database-generated GUID string on MS SQL Server and MySQL.

native

selects identity, sequence or
hilo depending upon the capabilities of the
underlying database.

assigned

lets the application assign an identifier to the object before
save() is called. This is the default strategy
if no <generator> element is specified.

select

retrieves a primary key, assigned by a database trigger, by selecting
the row by some unique key and retrieving the primary key value.

foreign

uses the identifier of another associated object. It is usually used in conjunction
with a <one-to-one> primary key association.

sequence-identity

a specialized sequence generation strategy that utilizes a
database sequence for the actual value generation, but combines
this with JDBC3 getGeneratedKeys to return the generated
identifier value as part of the insert statement execution. This
strategy is only supported on Oracle 10g drivers
targeted for JDK 1.4. Comments on these insert statements
are disabled due to a bug in the Oracle drivers.

5.1.4.2. Hi/lo algorithm

The hilo and seqhilo generators provide two alternate
implementations of the hi/lo algorithm. The
first implementation requires a "special" database table to hold the next available "hi" value.
Where supported, the second uses an Oracle-style sequence.

Unfortunately, you cannot use hilo when supplying your own
Connection to Hibernate. When Hibernate uses an application
server datasource to obtain connections enlisted with JTA, you must configure
the hibernate.transaction.manager_lookup_class.

5.1.4.3. UUID algorithm

The UUID contains: IP address, startup time of the JVM that is accurate to a quarter
second, system time and a counter value that is unique within the JVM. It is not
possible to obtain a MAC address or memory address from Java code, so this is
the best option without using JNI.

5.1.4.4. Identity columns and sequences

For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you
can use identity key generation. For databases that support
sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can use
sequence style key generation. Both of these strategies require
two SQL queries to insert a new object. For example:

For cross-platform development, the native strategy will, depending on the capabilities of the underlying database,
choose from the identity, sequence and
hilo strategies.

5.1.4.5. Assigned identifiers

If you want the application to assign identifiers, as opposed to having
Hibernate generate them, you can use the assigned generator.
This special generator uses the identifier value already assigned to the
object's identifier property. The generator is used when the primary key
is a natural key instead of a surrogate key. This is the default behavior
if you do not specify a <generator> element.

The assigned generator makes Hibernate use
unsaved-value="undefined". This forces Hibernate to go to
the database to determine if an instance is transient or detached, unless
there is a version or timestamp property, or you define
Interceptor.isUnsaved().

5.1.4.6. Primary keys assigned by triggers

Hibernate does not generate DDL with triggers. It is for legacy schemas only.

In the above example, there is a unique valued property named
socialSecurityNumber. It is defined by the class, as a
natural key and a surrogate key named person_id,
whose value is generated by a trigger.

5.1.5. Enhanced identifier generators

Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different
aspects of identifier generation. The first aspect is database portability; the second is optimization
Optimization means that you do not have to query the database for every request for a new identifier value. These two new
generators are intended to take the place of some of the named generators described above, starting
in 3.3.x. However, they are included in the current releases and can be referenced by FQN.

The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator
which is intended, firstly, as a replacement for the sequence generator and, secondly, as
a better portability generator than native. This is because native
generally chooses between identity and sequence which have
largely different semantics that can cause subtle issues in applications eyeing portability.
org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in
a different manner. It chooses between a table or a sequence in the database to store its
incrementing values, depending on the capabilities of the dialect being used. The difference between this
and native is that table-based and sequence-based storage have the same exact
semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-based
generators. This generator has a number of configuration parameters:

sequence_name (optional, defaults to hibernate_sequence):
the name of the sequence or table to be used.

initial_value (optional, defaults to 1): the initial
value to be retrieved from the sequence/table. In sequence creation terms, this is analogous
to the clause typically named "STARTS WITH".

increment_size (optional - defaults to 1): the value by
which subsequent calls to the sequence/table should differ. In sequence creation terms, this
is analogous to the clause typically named "INCREMENT BY".

force_table_use (optional - defaults to false): should
we force the use of a table as the backing structure even though the dialect might support
sequence?

value_column (optional - defaults to next_val): only
relevant for table structures, it is the name of the column on the table which is used to
hold the value.

The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which
is intended, firstly, as a replacement for the table generator, even though it actually
functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator, and secondly,
as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the
notion of pluggable optimizers. Essentially this generator defines a table capable of holding
a number of different increment values simultaneously by using multiple distinctly keyed rows. This
generator has a number of configuration parameters:

table_name (optional - defaults to hibernate_sequences):
the name of the table to be used.

value_column_name (optional - defaults to next_val):
the name of the column on the table that is used to hold the value.

segment_column_name (optional - defaults to sequence_name):
the name of the column on the table that is used to hold the "segment key". This is the
value which identifies which increment value to use.

segment_value (optional - defaults to default):
The "segment key" value for the segment from which we want to pull increment values for
this generator.

segment_value_length (optional - defaults to 255):
Used for schema generation; the column size to create this segment key column.

initial_value (optional - defaults to 1):
The initial value to be retrieved from the table.

increment_size (optional - defaults to 1):
The value by which subsequent calls to the table should differ.

5.1.6. Identifier generator optimization

For identifier generators that store values in the database, it is inefficient for them to hit the
database on each and every call to generate a new identifier value. Instead, you can
group a bunch of them in memory and only hit the database when you have exhausted your in-memory
value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators
(Section 5.1.5, “Enhanced identifier generators” support this operation.

none (generally this is the default if no optimizer was specified): this
will not perform any optimizations and hit the database for each and every request.

hilo: applies a hi/lo algorithm around the database retrieved values. The
values from the database for this optimizer are expected to be sequential. The values
retrieved from the database structure for this optimizer indicates the "group number". The
increment_size is multiplied by that value in memory to define a group
"hi value".

pooled: as with the case of hilo, this optimizer
attempts to minimize the number of hits to the database. Here, however, we simply store
the starting value for the "next group" into the database structure rather than a sequential
value in combination with an in-memory grouping algorithm. Here, increment_size
refers to the values coming from the database.

A table with a composite key can be mapped with multiple properties of the class
as identifier properties. The <composite-id> element
accepts <key-property> property mappings and
<key-many-to-one> mappings as child elements.

The persistent class must override equals()
and hashCode() to implement composite identifier equality. It must
also implement Serializable.

Unfortunately, this approach means that a persistent object
is its own identifier. There is no convenient "handle" other than the object itself.
You must instantiate an instance of the persistent class itself and populate its
identifier properties before you can load() the persistent state
associated with a composite key. We call this approach an embedded
composite identifier, and discourage it for serious applications.

A second approach is what we call a mapped composite identifier,
where the identifier properties named inside the <composite-id>
element are duplicated on both the persistent class and a separate identifier class.

In this example, both the composite identifier class, MedicareId,
and the entity class itself have properties named medicareNumber
and dependent. The identifier class must override
equals() and hashCode() and implement
Serializable. The main disadvantage of this approach is
code duplication.

The following attributes are used to specify a mapped composite identifier:

mapped (optional - defaults to false):
indicates that a mapped composite identifier is used, and that the contained
property mappings refer to both the entity class and the composite identifier
class.

class (optional - but required for a mapped composite identifier):
the class used as a composite identifier.

We will describe a third, even more convenient approach, where the composite identifier
is implemented as a component class in Section 8.4, “Components as composite identifiers”. The
attributes described below apply only to this alternative approach:

name (optional - required for this approach): a property of
component type that holds the composite identifier. Please see chapter 9 for more information.

class (optional - defaults to the property type determined by
reflection): the component class used as a composite identifier. Please see the next section for more information.

The third approach, an identifier component, is recommended
for almost all applications.

5.1.8. Discriminator

The <discriminator> element is required for polymorphic persistence
using the table-per-class-hierarchy mapping strategy. It declares a discriminator column of the
table. The discriminator column contains marker values that tell the persistence layer what
subclass to instantiate for a particular row. A restricted set of types can be used:
string, character, integer,
byte, short, boolean,
yes_no, true_false.

unsaved-value (optional - defaults to undefined):
a version property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or loaded
in a previous session. Undefined specifies that the identifier
property value should be used.

generated (optional - defaults to never):
specifies that this version property value is generated by the database.
See the discussion of generated properties for more information.

insert (optional - defaults to true):
specifies whether the version column should be included in SQL insert statements.
It can be set to false if the database column
is defined with a default value of 0.

Version numbers can be of Hibernate type long, integer,
short, timestamp or calendar.

A version or timestamp property should never be null for a detached instance.
Hibernate will detect any instance with a null version or timestamp as transient,
irrespective of what other unsaved-value strategies are specified.
Declaring a nullable version or timestamp property is an easy way to avoid
problems with transitive reattachment in Hibernate. It is especially useful for people
using assigned identifiers or composite keys.

5.1.10. Timestamp (optional)

The optional <timestamp> element indicates that the table contains
timestamped data. This provides an alternative to versioning. Timestamps are
a less safe implementation of optimistic locking. However, sometimes the application might
use the timestamps in other ways.

unsaved-value (optional - defaults to null):
a version property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or loaded
in a previous session. Undefined specifies that the identifier
property value should be used.

source (optional - defaults to vm):
Where should Hibernate retrieve the timestamp value from? From the database,
or from the current JVM? Database-based timestamps incur an overhead because
Hibernate must hit the database in order to determine the "next value".
It is safer to use in clustered environments. Not
all Dialects are known to support the retrieval of the
database's current timestamp. Others may also be unsafe for usage
in locking due to lack of precision (Oracle 8, for example).

generated (optional - defaults to never):
specifies that this timestamp property value is actually generated by the database.
See the discussion of generated properties for more information.

Note

<Timestamp> is equivalent to
<version type="timestamp">. And
<timestamp source="db"> is equivalent to
<version type="dbtimestamp">

5.1.11. Property

The <property> element declares a persistent JavaBean style
property of the class.

column (optional - defaults to the property name): the name
of the mapped database table column. This can also be specified by nested
<column> element(s).

type (optional): a name that indicates the Hibernate type.

update, insert (optional - defaults to true):
specifies that the mapped columns should be included in SQL UPDATE
and/or INSERT statements. Setting both to false
allows a pure "derived" property whose value is initialized from some other
property that maps to the same column(s), or by a trigger or other application.

formula (optional): an SQL expression that defines the value for a
computed property. Computed properties do not have a column
mapping of their own.

lazy (optional - defaults to false): specifies
that this property should be fetched lazily when the instance variable is first
accessed. It requires build-time bytecode instrumentation.

unique (optional): enables the DDL generation of a unique
constraint for the columns. Also, allow this to be the target of
a property-ref.

not-null (optional): enables the DDL generation of a nullability
constraint for the columns.

optimistic-lock (optional - defaults to true):
specifies that updates to this property do or do not require acquisition of the
optimistic lock. In other words, it determines if a version increment should occur when
this property is dirty.

generated (optional - defaults to never):
specifies that this property value is actually generated by the database.
See the discussion of generated properties for more information.

The name of a Java class with a default basic type: int, float,
char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob etc.

The name of a serializable Java class.

The class name of a custom type: com.illflow.type.MyCustomType etc.

If you do not specify a type, Hibernate will use reflection upon the named
property and guess the correct Hibernate type. Hibernate will attempt to
interpret the name of the return class of the property getter using, in order, rules 2, 3,
and 4.
In certain cases you will need the type
attribute. For example, to distinguish between Hibernate.DATE and
Hibernate.TIMESTAMP, or to specify a custom type.

The access attribute allows you to control how Hibernate accesses
the property at runtime. By default, Hibernate will call the property get/set pair.
If you specify access="field", Hibernate will bypass the get/set
pair and access the field directly using reflection. You can specify your own
strategy for property access by naming a class that implements the interface
org.hibernate.property.PropertyAccessor.

A powerful feature is derived properties. These properties are by
definition read-only. The property value is computed at load time. You declare
the computation as an SQL expression. This then translates to a SELECT
clause subquery in the SQL query that loads an instance:

You can reference the entity table by not declaring an alias on
a particular column. This would be customerId in the given example. You can also use
the nested <formula> mapping element
if you do not want to use the attribute.

5.1.12. Many-to-one

An ordinary association to another persistent class is declared using a
many-to-one element. The relational model is a
many-to-one association; a foreign key in one table is referencing
the primary key column(s) of the target table.

update, insert (optional - defaults to true):
specifies that the mapped columns should be included in SQL UPDATE
and/or INSERT statements. Setting both to false
allows a pure "derived" association whose value is initialized from another
property that maps to the same column(s), or by a trigger or other application.

property-ref (optional): the name of a property of the associated
class that is joined to this foreign key. If not specified, the primary key of
the associated class is used.

unique (optional): enables the DDL generation of a unique
constraint for the foreign-key column. By allowing this to be the target of
a property-ref, you can make the association multiplicity
one-to-one.

not-null (optional): enables the DDL generation of a nullability
constraint for the foreign key columns.

optimistic-lock (optional - defaults to true):
specifies that updates to this property do or do not require acquisition of the
optimistic lock. In other words, it determines if a version increment should occur when
this property is dirty.

lazy (optional - defaults to proxy):
by default, single point associations are proxied. lazy="no-proxy"
specifies that the property should be fetched lazily when the instance variable
is first accessed. This requires build-time bytecode instrumentation.
lazy="false" specifies that the association will always
be eagerly fetched.

not-found (optional - defaults to exception):
specifies how foreign keys that reference missing rows will be handled.
ignore will treat a missing row as a null association.

entity-name (optional): the entity name of the associated class.

formula (optional): an SQL expression that defines the value for a
computed foreign key.

Setting a value of the cascade attribute to any meaningful
value other than none will propagate certain operations to the
associated object. The meaningful values are divided into three categories. First, basic
operations, which include: persist, merge, delete, save-update, evict, replicate, lock and
refresh; second, special values: delete-orphan;
and third, all comma-separated combinations of operation
names: cascade="persist,merge,evict" or
cascade="all,delete-orphan". See Section 10.11, “Transitive persistence”
for a full explanation. Note that single valued, many-to-one and
one-to-one, associations do not support orphan delete.

Here is an example of a typical many-to-one declaration:

<many-to-onename="product"class="Product"column="PRODUCT_ID"/>

The property-ref attribute should only be used for mapping legacy
data where a foreign key refers to a unique key of the associated table other than
the primary key. This is a complicated and confusing relational model. For example, if the
Product class had a unique serial number that is not the primary
key. The unique attribute controls Hibernate's DDL generation with
the SchemaExport tool.

class (optional - defaults to the property type
determined by reflection): the name of the associated class.

cascade (optional): specifies which operations should
be cascaded from the parent object to the associated object.

constrained (optional): specifies that a foreign key constraint
on the primary key of the mapped table and references the table of the associated
class. This option affects the order in which save() and
delete() are cascaded, and determines whether the association
can be proxied. It is also used by the schema export tool.

formula (optional): almost all one-to-one associations map to the
primary key of the owning entity. If this is not the case, you can
specify another column, columns or expression to join on using an SQL formula. See
org.hibernate.test.onetooneformula for an example.

lazy (optional - defaults to proxy):
by default, single point associations are proxied. lazy="no-proxy"
specifies that the property should be fetched lazily when the instance variable
is first accessed. It requires build-time bytecode instrumentation.
lazy="false" specifies that the association will always
be eagerly fetched. Note that if constrained="false",
proxying is impossible and Hibernate will eagerly fetch the association.

entity-name (optional): the entity name of the associated class.

There are two varieties of one-to-one associations:

primary key associations

unique foreign key associations

Primary key associations do not need an extra table column. If two rows are related by
the association, then the two table rows share the same primary key value.
To relate two objects by a primary key association, ensure that they
are assigned the same identifier value.

For a primary key association, add the following mappings to Employee and
Person respectively:

<one-to-onename="person"class="Person"/>

<one-to-onename="employee"class="Employee"constrained="true"/>

Ensure that the primary keys of the related rows in the PERSON and
EMPLOYEE tables are equal. You use a special Hibernate identifier generation strategy
called foreign:

5.1.14. Natural-id

Although we recommend the use of surrogate keys as primary keys, you should try
to identify natural keys for all entities. A natural key is a property or combination of
properties that is unique and non-null. It is also immutable. Map the
properties of the natural key inside the <natural-id> element.
Hibernate will generate the necessary unique key and nullability constraints and, as a result, your
mapping will be more self-documenting.

It is recommended that you implement equals() and
hashCode() to compare the natural key properties of the entity.

This mapping is not intended for use with entities that have natural primary keys.

5.1.15. Component and dynamic-component

The <component> element maps properties of a
child object to columns of the table of a parent class. Components can, in
turn, declare their own properties, components or collections. See
the "Component" examples below:

lazy (optional - defaults to false): specifies
that this component should be fetched lazily when the instance variable is first
accessed. It requires build-time bytecode instrumentation.

optimistic-lock (optional - defaults to true):
specifies that updates to this component either do or do not require acquisition of the
optimistic lock. It determines if a version increment should occur when
this property is dirty.

unique (optional - defaults to false):
specifies that a unique constraint exists upon all mapped columns of the
component.

The child <property> tags map properties of the
child class to table columns.

The <component> element allows a <parent>
subelement that maps a property of the component class as a reference back to the
containing entity.

The <dynamic-component> element allows a Map
to be mapped as a component, where the property names refer to keys of the map. See
Section 8.5, “Dynamic components” for more information.

5.1.16. Properties

The <properties> element allows the definition of a named,
logical grouping of the properties of a class. The most important use of the construct
is that it allows a combination of properties to be the target of a
property-ref. It is also a convenient way to define a multi-column
unique constraint. For example:

name: the logical name of the grouping. It is
not an actual property name.

insert: do the mapped columns appear in SQL
INSERTs?

update: do the mapped columns appear in SQL
UPDATEs?

optimistic-lock (optional - defaults to true):
specifies that updates to these properties either do or do not require acquisition of the
optimistic lock. It determines if a version increment should occur when
these properties are dirty.

unique (optional - defaults to false):
specifies that a unique constraint exists upon all mapped columns of the
component.

Each subclass declares its own persistent properties and subclasses.
<version> and <id> properties
are assumed to be inherited from the root class. Each subclass in a hierarchy must
define a unique discriminator-value. If this is not specified, the
fully qualified Java class name is used.

5.1.18. Joined-subclass

Each subclass can also be mapped to its own table. This is called the table-per-subclass
mapping strategy. An inherited state is retrieved by joining with the table of the
superclass. To do this you use the <joined-subclass> element. For example:

proxy (optional): specifies a class or interface to use
for lazy initializing proxies.

lazy (optional, defaults to true): setting
lazy="false" disables the use of lazy fetching.

A discriminator column is not required for this mapping strategy. Each subclass must,
however, declare a table column holding the object identifier using the
<key> element. The mapping at the start of the chapter
would then be re-written as:

5.1.19. Union-subclass

A third option is to map only the concrete classes of an inheritance hierarchy
to tables. This is called the table-per-concrete-class strategy. Each table defines all
persistent states of the class, including the inherited state. In Hibernate, it is
not necessary to explicitly map such inheritance hierarchies. You
can map each class with a separate <class>
declaration. However, if you wish use polymorphic associations (e.g. an association
to the superclass of your hierarchy), you need to
use the <union-subclass> mapping. For example:

schema (optional): overrides the schema name specified by
the root <hibernate-mapping> element.

catalog (optional): overrides the catalog name specified by
the root <hibernate-mapping> element.

fetch (optional - defaults to join):
if set to join, the default, Hibernate will use an inner join
to retrieve a <join> defined by a class or its superclasses. It will use
an outer join for a <join> defined by a subclass.
If set to select then Hibernate will use a sequential select for
a <join> defined on a subclass. This will be issued only
if a row represents an instance of the subclass. Inner joins will still
be used to retrieve a <join> defined by the class and its
superclasses.

inverse (optional - defaults to false):
if enabled, Hibernate will not insert or update the properties defined
by this join.

optional (optional - defaults to false):
if enabled, Hibernate will insert a row only if the properties defined by this
join are non-null. It will always use an outer join to retrieve the properties.

For example, address information for a person can be mapped to a separate
table while preserving value type semantics for all properties:

This feature is often only useful for legacy data models. We recommend fewer
tables than classes and a fine-grained domain model. However, it is useful
for switching between inheritance mapping strategies in a single hierarchy, as
explained later.

5.1.21. Key

The <key> element has featured a few times within this guide.
It appears anywhere the parent mapping element defines a join to
a new table that references
the primary key of the original table. It also defines the foreign key in the joined table:

property-ref (optional): specifies that the foreign key refers
to columns that are not the primary key of the original table. It is provided for
legacy data.

not-null (optional): specifies that the foreign key columns
are not nullable. This is implied whenever the foreign key is also part of the
primary key.

update (optional): specifies that the foreign key should never
be updated. This is implied whenever the foreign key is also part of the primary
key.

unique (optional): specifies that the foreign key should have
a unique constraint. This is implied whenever the foreign key is also the primary key.

For systems where delete performance is important, we recommend that all keys should be
defined on-delete="cascade". Hibernate uses a database-level
ON CASCADE DELETE constraint, instead of many individual
DELETE statements. Be aware that this feature bypasses Hibernate's
usual optimistic locking strategy for versioned data.

The not-null and update attributes are useful when
mapping a unidirectional one-to-many association. If you map a unidirectional one-to-many association
to a non-nullable foreign key, you must declare the key column using
<key not-null="true">.

5.1.22. Column and formula elements

Mapping elements which accept a column attribute will alternatively
accept a <column> subelement. Likewise, <formula>
is an alternative to the formula attribute. For example:

Most of the attributes on column provide a means of tailoring the
DDL during automatic schema generation. The read and write
attributes allow you to specify custom SQL that Hibernate will use to access the column's value.
For more on this, see the discussion of
column read and write expressions.

The column and formula elements can even be combined
within the same property or association mapping to express, for example, exotic join
conditions.

5.1.23. Import

If your application has two persistent classes with the same name, and you do not want to
specify the fully qualified package name in Hibernate queries, classes can be "imported"
explicitly, rather than relying upon auto-import="true". You can also import
classes and interfaces that are not explicitly mapped:

<importclass="java.lang.Object"rename="Universe"/>

<import
class="ClassName"
rename="ShortName"
/>

class: the fully qualified class name of any Java class.

rename (optional - defaults to the unqualified class name):
a name that can be used in the query language.

5.1.24. Any

There is one more type of property mapping. The <any> mapping element
defines a polymorphic association to classes from multiple tables. This type of mapping
requires more than one column. The first column contains the type of the associated entity.
The remaining columns contain the identifier. It is impossible to specify a foreign key constraint
for this kind of association. This is not the usual way of mapping
polymorphic associations and you should use this only in special cases. For example, for audit logs,
user session data, etc.

The meta-type attribute allows the application to specify a custom type that
maps database column values to persistent classes that have identifier properties of the
type specified by id-type. You must specify the mapping from values of
the meta-type to class names.

optimistic-lock (optional - defaults to true):
specifies that updates to this property either do or do not require acquisition of the
optimistic lock. It defines whether a version increment should occur if this
property is dirty.

5.2. Hibernate types

5.2.1. Entities and values

In relation to the persistence service, Java language-level objects are classified
into two groups:

An entity exists independently of any other objects holding
references to the entity. Contrast this with the usual Java model, where an
unreferenced object is garbage collected. Entities must be explicitly saved and
deleted. Saves and deletions, however, can be cascaded
from a parent entity to its children. This is different from the ODMG model of
object persistence by reachability and corresponds more closely to how
application objects are usually used in large systems. Entities support
circular and shared references. They can also be versioned.

An entity's persistent state consists of references to other entities and
instances of value types. Values are primitives:
collections (not what is inside a collection), components and certain immutable
objects. Unlike entities, values in particular collections and components,
are persisted and deleted by reachability. Since value
objects and primitives are persisted and deleted along with their containing
entity, they cannot be independently versioned. Values have no independent
identity, so they cannot be shared by two entities or collections.

Until now, we have been using the term "persistent class" to refer to
entities. We will continue to do that. Not all
user-defined classes with a persistent state, however, are entities. A
component is a user-defined class with value semantics.
A Java property of type java.lang.String also has value
semantics. Given this definition, all types (classes) provided
by the JDK have value type semantics in Java, while user-defined types can
be mapped with entity or value type semantics. This decision is up to the
application developer. An entity class in a domain model will normally have
shared references to a single instance of that class, while composition or
aggregation usually translates to a value type.

We will revisit both concepts throughout this reference guide.

The challenge is to map the Java type system, and the developers' definition of
entities and value types, to the SQL/database type system. The bridge between
both systems is provided by Hibernate. For entities,
<class>, <subclass> and so on are used.
For value types we use <property>,
<component>etc., that usually have a type
attribute. The value of this attribute is the name of a Hibernate
mapping type. Hibernate provides a range of mappings for standard
JDK value types out of the box. You can write your own mapping types and implement your own
custom conversion strategies.

With the exception of collections, all built-in Hibernate types support null semantics.

5.2.2. Basic value types

The built-in basic mapping types can be roughly categorized into the following:

Type mappings from Java primitives or wrapper classes to appropriate
(vendor-specific) SQL column types. boolean, yes_no
and true_false are all alternative encodings for
a Java boolean or java.lang.Boolean.

string

A type mapping from java.lang.String to
VARCHAR (or Oracle VARCHAR2).

date, time, timestamp

Type mappings from java.util.Date and its subclasses
to SQL types DATE, TIME and
TIMESTAMP (or equivalent).

Type mappings from java.util.Locale,
java.util.TimeZone and
java.util.Currency
to VARCHAR (or Oracle VARCHAR2).
Instances of Locale and Currency are
mapped to their ISO codes. Instances of TimeZone are
mapped to their ID.

class

A type mapping from java.lang.Class to
VARCHAR (or Oracle VARCHAR2).
A Class is mapped to its fully qualified name.

binary

Maps byte arrays to an appropriate SQL binary type.

text

Maps long Java strings to a SQL CLOB or
TEXT type.

serializable

Maps serializable Java types to an appropriate SQL binary type. You
can also indicate the Hibernate type serializable with
the name of a serializable Java class or interface that does not default
to a basic type.

clob, blob

Type mappings for the JDBC classes java.sql.Clob and
java.sql.Blob. These types can be inconvenient for some
applications, since the blob or clob object cannot be reused outside of
a transaction. Driver support is patchy and inconsistent.

Type mappings for what are considered mutable Java types. This is where
Hibernate makes certain optimizations appropriate only for immutable
Java types, and the application treats the object as immutable. For
example, you should not call Date.setTime() for an
instance mapped as imm_timestamp. To change the
value of the property, and have that change made persistent, the
application must assign a new, nonidentical, object to the property.

Unique identifiers of entities and collections can be of any basic type except
binary, blob and clob.
Composite identifiers are also allowed. See below for more information.

5.2.3. Custom value types

It is relatively easy for developers to create their own value types. For example,
you might want to persist properties of type java.lang.BigInteger
to VARCHAR columns. Hibernate does not provide a built-in type
for this. Custom types are not limited to mapping a property, or collection element,
to a single table column. So, for example, you might have a Java property
getName()/setName() of type
java.lang.String that is persisted to the columns
FIRST_NAME, INITIAL, SURNAME.

To implement a custom type, implement either org.hibernate.UserType
or org.hibernate.CompositeUserType and declare properties using the
fully qualified classname of the type. View
org.hibernate.test.DoubleStringType to see the kind of things that
are possible.

Notice the use of <column> tags to map a property to multiple
columns.

The CompositeUserType, EnhancedUserType,
UserCollectionType, and UserVersionType
interfaces provide support for more specialized uses.

You can even supply parameters to a UserType in the mapping file. To
do this, your UserType must implement the
org.hibernate.usertype.ParameterizedType interface. To supply parameters
to your custom type, you can use the <type> element in your mapping
files.

The UserType can now retrieve the value for the parameter named
default from the Properties object passed to it.

If you regularly use a certain UserType, it is useful to define a
shorter name for it. You can do this using the <typedef> element.
Typedefs assign a name to a custom type, and can also contain a list of default
parameter values if the type is parameterized.

It is also possible to override the parameters supplied in a typedef on a case-by-case basis
by using type parameters on the property mapping.

Even though Hibernate's rich range of built-in types and support for components means you
will rarely need to use a custom type, it is
considered good practice to use custom types for non-entity classes that occur frequently
in your application. For example, a MonetaryAmount class is a good
candidate for a CompositeUserType, even though it could be mapped
as a component. One reason for this is abstraction. With a custom type, your mapping
documents would be protected against changes to the way
monetary values are represented.

5.3. Mapping a class more than once

It is possible to provide more than one mapping for a particular persistent class. In this
case, you must specify an entity name to disambiguate between instances
of the two mapped entities. By default, the entity name is the same as the class name.
Hibernate lets you specify the entity name when working with persistent objects, when writing
queries, or when mapping associations to the named entity.

5.4. SQL quoted identifiers

You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or
column name in backticks in the mapping document. Hibernate will use the correct quotation
style for the SQL Dialect. This is usually double quotes, but the SQL
Server uses brackets and MySQL uses backticks.

5.5. Metadata alternatives

XML does not suit all users so there are some alternative ways to define O/R mapping metadata in Hibernate.

5.5.1. Using XDoclet markup

Many Hibernate users prefer to embed mapping information directly in sourcecode using
XDoclet @hibernate.tags. We do not cover this approach in this
reference guide since it is considered part of XDoclet. However, we include the
following example of the Cat class with XDoclet mappings:

5.5.2. Using JDK 5.0 Annotations

JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe and
checked at compile time. This mechanism is more powerful than XDoclet annotations and
better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion
and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification
(JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans.
Hibernate3 implements the EntityManager of JSR-220 (the persistence API).
Support for mapping metadata is available via the Hibernate Annotations
package as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.

Note

Support for JDK 5.0 Annotations (and JSR-220) is currently under development.
Please refer to the Hibernate Annotations module for more details.

5.6. Generated properties

Generated properties are properties that have their values generated by the
database. Typically, Hibernate applications needed to refresh
objects that contain any properties for which the database was generating values.
Marking properties as generated, however, lets the application delegate this
responsibility to Hibernate. When Hibernate issues an SQL INSERT
or UPDATE for an entity that has defined generated properties, it immediately
issues a select afterwards to retrieve the generated values.

never (the default): the given property value
is not generated within the database.

insert: the given property value is generated on
insert, but is not regenerated on subsequent updates. Properties like created-date
fall into this category. Even though
version and
timestamp properties can
be marked as generated, this option is not available.

always: the property value is generated both
on insert and on update.

5.7. Column read and write expressions

Hibernate allows you to customize the SQL it uses to read and write the values
of columns mapped to simple properties.
For example, if your database provides a set of data encryption functions, you can
invoke them for individual columns like this:

Hibernate applies the custom expressions automatically whenever the property is
referenced in a query. This functionality is similar to a derived-property
formula with two differences:

The property is backed by one or more columns that are exported as part of automatic
schema generation.

The property is read-write, not read-only.

The write expression, if specified, must contain exactly one '?' placeholder
for the value.

5.8. Auxiliary database objects

Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with
Hibernate's schema evolution tools, they have the ability to fully define
a user schema within the Hibernate mapping files. Although designed specifically
for creating and dropping things like triggers or stored procedures, any
SQL command that can be run via a java.sql.Statement.execute()
method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for
defining auxiliary database objects:

The first mode is to explicitly list the CREATE and DROP commands in the mapping
file:

The actual interface might be java.util.Set,
java.util.Collection, java.util.List,
java.util.Map, java.util.SortedSet,
java.util.SortedMap or anything you like
("anything you like" means you will have to write an implementation of
org.hibernate.usertype.UserCollectionType.)

Notice how the instance variable was initialized with an instance of
HashSet. This is the best way to initialize collection
valued properties of newly instantiated (non-persistent) instances. When
you make the instance persistent, by calling persist()
for example, Hibernate will actually replace the HashSet
with an instance of Hibernate's own implementation of Set.
Be aware of the following errors:

The persistent collections injected by Hibernate behave like
HashMap, HashSet,
TreeMap, TreeSet or
ArrayList, depending on the interface type.

Collections instances have the usual behavior of value types. They are
automatically persisted when referenced by a persistent object and
are automatically deleted when unreferenced. If a collection is passed from one
persistent object to another, its elements might be moved from one table to
another. Two entities cannot share a reference to the same collection
instance. Due to the underlying relational model, collection-valued properties
do not support null value semantics. Hibernate does not distinguish between
a null collection reference and an empty collection.

Use persistent collections
the same way you use ordinary Java collections. However, please ensure you understand
the semantics of bidirectional associations (these are discussed later).

6.2. Collection mappings

Tip

There are quite a range of mappings that can be generated for collections that cover
many common relational models. We suggest you experiment with the schema generation tool
so that you understand how various mapping declarations translate to database tables.

The Hibernate mapping element used for mapping a collection depends upon
the type of interface. For example, a <set>
element is used for mapping properties of type Set.

table (optional - defaults to property name): the
name of the collection table. It is not used for one-to-many associations.

schema (optional): the name of a table schema to
override the schema declared on the root element

lazy (optional - defaults to true):
disables lazy fetching and specifies that the association is
always eagerly fetched. It can also be used to enable "extra-lazy" fetching where most
operations do not initialize the collection. This is suitable for large
collections.

inverse (optional - defaults to false):
marks this collection as the "inverse" end of a bidirectional association.

sort (optional): specifies a sorted collection with
natural sort order or a given comparator class.

order-by (optional, JDK1.4 only): specifies a table column or columns
that define the iteration order of the Map, Set
or bag, together with an optional asc or desc.

where (optional): specifies an arbitrary SQL WHERE
condition that is used when retrieving or removing the collection. This is useful if the
collection needs to contain only a subset of the available data.

optimistic-lock (optional - defaults to true):
specifies that changes to the state of the collection results in increments of the
owning entity's version. For one-to-many associations you may want to
disable this setting.

mutable (optional - defaults to true):
a value of false specifies that the elements of the
collection never change. This allows for minor performance optimization in some cases.

6.2.1. Collection foreign keys

Collection instances are distinguished in the database by the foreign key of
the entity that owns the collection. This foreign key is referred to as the
collection key column, or columns, of the collection
table. The collection key column is mapped by the <key>
element.

There can be a nullability constraint on the foreign key column. For most
collections, this is implied. For unidirectional one-to-many associations,
the foreign key column is nullable by default, so you may need to specify
not-null="true".

<keycolumn="productSerialNumber"not-null="true"/>

The foreign key constraint can use ON DELETE CASCADE.

<keycolumn="productSerialNumber"on-delete="cascade"/>

See the previous chapter for a full definition of the <key>
element.

6.2.2. Collection elements

Collections can contain almost any other Hibernate type, including: basic types,
custom types, components and references to other entities. This is an
important distinction. An object in a collection might be handled with "value"
semantics (its life cycle fully depends on the collection owner), or it might be a
reference to another entity with its own life cycle. In the latter case, only the
"link" between the two objects is considered to be a state held by the collection.

The contained type is referred to as the collection element type.
Collection elements are mapped by <element> or
<composite-element>, or in the case of entity references,
with <one-to-many> or <many-to-many>.
The first two map elements with value semantics, the next two are used to map entity
associations.

6.2.3. Indexed collections

All collection mappings, except those with set and bag semantics, need an
index column in the collection table. An index column is a column that maps to an
array index, or List index, or Map key. The
index of a Map may be of any basic type, mapped with
<map-key>. It can be an entity reference mapped with
<map-key-many-to-many>, or it can be a composite type
mapped with <composite-map-key>. The index of an array or
list is always of type integer and is mapped using the
<list-index> element. The mapped column contains
sequential integers that are numbered from zero by default.

<list-index
column="column_name"
base="0|1|..."/>

column_name (required): the name of the column holding the
collection index values.

base (optional - defaults to 0): the value
of the index column that corresponds to the first element of the list or array.

column (optional): the name of the foreign key
column for the collection index values.

formula (optional): a SQ formula used to evaluate the
foreign key of the map key.

class (required): the entity class used as the map key.

If your table does not have an index column, and you still wish to use List
as the property type, you can map the property as a Hibernate <bag>.
A bag does not retain its order when it is retrieved from the database, but it can be
optionally sorted or ordered.

6.2.4. Collections of values and many-to-many associations

Any collection of values or many-to-many associations requires a dedicated
collection table with a foreign key column or columns,
collection element column or columns, and possibly
an index column or columns.

formula (optional): an SQL formula used to evaluate the element
foreign key value.

class (required): the name of the associated class.

fetch (optional - defaults to join):
enables outer-join or sequential select fetching for this association. This
is a special case; for full eager fetching in a single SELECT
of an entity and its many-to-many relationships to other entities, you would
enable join fetching,not only of the collection itself,
but also with this attribute on the <many-to-many>
nested element.

unique (optional): enables the DDL generation of a unique
constraint for the foreign-key column. This makes the association multiplicity
effectively one-to-many.

not-found (optional - defaults to exception):
specifies how foreign keys that reference missing rows will be handled:
ignore will treat a missing row as a null association.

entity-name (optional): the entity name of the associated class,
as an alternative to class.

property-ref (optional): the name of a property of the associated
class that is joined to this foreign key. If not specified, the primary key of
the associated class is used.

not-found (optional - defaults to exception):
specifies how cached identifiers that reference missing rows will be handled.
ignore will treat a missing row as a null association.

entity-name (optional): the entity name of the associated class,
as an alternative to class.

The <one-to-many> element does not need to
declare any columns. Nor is it necessary to specify the table
name anywhere.

Warning

If the foreign key column of a
<one-to-many> association is declared NOT NULL,
you must declare the <key> mapping
not-null="true" or use a bidirectional association
with the collection mapping marked inverse="true". See the discussion
of bidirectional associations later in this chapter for more information.

The following example shows a map of Part entities by name, where
partName is a persistent property of Part.
Notice the use of a formula-based index:

If you want the database itself to order the collection elements, use the
order-by attribute of set, bag
or map mappings. This solution is only available under
JDK 1.4 or higher and is implemented using LinkedHashSet or
LinkedHashMap. This performs the ordering in the SQL query and
not in the memory.

Changes made only to the inverse end of the association are not
persisted. This means that Hibernate has two representations in memory for every
bidirectional association: one link from A to B and another link from B to A. This
is easier to understand if you think about the Java object model and how
a many-to-many relationship in Javais created:

category.getItems().add(item);//The category now "knows" about the relationshipitem.getCategories().add(category);//The item now "knows" about the relationshipsession.persist(item);//The relationship won't be saved!session.persist(category);//The relationship will be saved

The non-inverse side is used to save the in-memory representation to the database.

You can define a bidirectional one-to-many association by mapping a one-to-many association
to the same table column(s) as a many-to-one association and declaring the many-valued
end inverse="true".

Mapping one end of an association with inverse="true" does not
affect the operation of cascades as these are orthogonal concepts.

6.3.3. Bidirectional associations with indexed collections

A bidirectional association where one end is represented as a <list>
or <map>, requires special consideration. If there is a property of
the child class that maps to the index column you can use
inverse="true" on the collection mapping:

If there is no such property on the child class, the association cannot be considered
truly bidirectional. That is, there is information available at one end of the association that is
not available at the other end. In this case, you cannot map the collection
inverse="true". Instead, you could use the following mapping:

A second approach is to remodel the association as an entity class. This
is the most common approach.

A final alternative is to use composite elements, which will be discussed later.

6.3.5. Using an <idbag>

The majority of the many-to-many associations and collections of values
shown previously all map to tables with composite keys, even though it has been have suggested
that entities should have synthetic identifiers (surrogate keys). A
pure association table does not seem to benefit much from
a surrogate key, although a collection of composite values might.
It is for this reason that Hibernate provides a feature that allows you to map many-to-many
associations and collections of values to a table with a surrogate key.

The <idbag> element lets you map a List
(or Collection) with bag semantics. For example:

An <idbag> has a synthetic id generator,
just like an entity class. A different surrogate key is assigned to each collection
row. Hibernate does not, however, provide any mechanism for discovering the surrogate key value
of a particular row.

The update performance of an <idbag> supersedes a regular <bag>.
Hibernate can locate individual rows efficiently and update or delete them
individually, similar to a list, map or set.

In the current implementation, the native identifier generation
strategy is not supported for <idbag> collection identifiers.

7.1. Introduction

Association mappings are often the most difficult thing to implement correctly. In
this section we examine some canonical cases one by one, starting
with unidirectional mappings and then bidirectional cases.
We will use Person and Address in all
the examples.

Associations will be classified by multiplicity and whether or not they map to an intervening
join table.

Nullable foreign keys are not considered to be good practice in traditional data
modelling, so our examples do not use nullable foreign keys. This is not a
requirement of Hibernate, and the mappings will work if you drop the
nullability constraints.

7.2. Unidirectional associations

7.2.1. Many-to-one

A unidirectional many-to-one association is the most
common kind of unidirectional association.

If you use a List, or other indexed collection,
set the key column of the foreign key to not null.
Hibernate will manage the association from the collections side to maintain the index
of each element, making the other side virtually inverse by setting
update="false" and insert="false":

If the underlying foreign key column is NOT NULL, it
is important that you define not-null="true" on the
<key> element of the collection mapping.
Do not only
declare not-null="true" on a possible nested
<column> element, but on the <key>
element.